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| | + | <table width="40%"><tr><td class="l1"> </td><td> |
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| − | __TOC__
| + | ===Hardware=== |
| − | | + | <table width="100%"> |
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| + | <tr class="s1"><td class="l1">High performance computing <!-- (... at the bench: GPUs, FPGAs, Clusters) --></td></tr> |
| | + | <tr class="s2"><td class="l1">Cloud computing</td></tr> |
| | + | <tr><td class="sp"> </td></tr> |
| | + | </table> |
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| − | <div style="padding: 5px; background: #A6AFD0; border:solid 1px #AAAAAA; font-size:200%;font-weight:bold;"> | + | ===Systems and Tools=== |
| − | Assignment 4 - Homology modeling
| + | <table width="100%"> |
| − | </div>
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| − | <div style="padding: 15px; background: #F0F1F7; border:solid 1px #AAAAAA; font-size:125%;color:#444444"> | + | <tr class="s1"><td class="l1 mw-collapsible mw-collapsed" data-expandtext="Expand subtopics" data-collapsetext="Collapse">[[Unix]] |
| − | ;How could the search for ultimate truth have revealed so hideous and visceral-looking an object?
| + | <div class="mw-collapsible-content"> |
| − | ::''<small>Max Perutz (on his first glimpse of the Hemoglobin structure)</small>''
| + | <table width="100%"><tr class="s2"><td class="l2">[[Unix system administration]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Unix automation]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Program installation]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[wget]]</td></tr></table> |
| | </div> | | </div> |
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| + | </td></tr> |
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| − | Where is the hidden beauty in structure, and where, the "ultimate truth"? In the previous assignments we have studied sequence conservation in APSES family domains and we have seen homologues in all fungal species. This is an ancient protein family that had already duplicated to several paralogues at the time the cenancestor of all fungi lived, more than 600,000,000 years ago, in the [http://www.ucmp.berkeley.edu/fungi/fungifr.html Vendian period] of the Proterozoic era of Precambrian times.
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| − | In order to understand how specific residues in the sequence contribute to the putative function of the protein, and why and how they are conserved throughout evolution, we would need to study an explicit molecular model of an APSES domain protein, bound to its cognate DNA sequence. Explanations of a protein's observed properties and functions can't rely on the general fact that it binds DNA, we need to consider details in terms of specific residues and their spatial arrangement. In particular, it would be interesting to correlate the conservation patterns of key residues with their potential to make specific DNA binding interactions. Unfortunately, no APSES domain structures in complex with bound DNA has been solved up to now, and the experimental evidence we have considered in Assignment 2 ([http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=10747782 Taylor ''et al.'', 2000]) is not sufficient to unambiguously define the details of how a DNA double helix might be bound. Moreover, at least two distinct modes of DNA binding are known for proteins of the winged-helix superfamily, of which the APSES domain is a member.
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| − | ''In this assignment you will (1) construct a molecular model of the Mbp1 orthologue in your assigned organism, (2) identify similar structures of distantly related domains for which protein-DNA complexes are known, (3) define whether the available evidence allows you to distinguish between different modes of ligand binding, and (4) assemble a hypothetical complex structure.''
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| − | For the following, please remember the following terminology:
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| − | | |
| − | ;Target
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| − | :The protein that you are planning to model.
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| − | ;Template
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| − | :The protein whose structure you are using as a guide to build the model.
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| − | ;Model
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| − | :The structure that results from the modeling process. It has the '''Target sequence''' and is similar to the '''Template structure'''.
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| − | A brief overview article on the construction and use of homology models is linked to the resource section at the bottom of this page. That section also contains links to other sites and resources you might require.
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| − | {{Template:Preparation|
| + | <tr class="s2"><td class="l1">[[Network Configuration]]</td></tr> |
| − | care=Be sure you have understood all parts of the assignment and cover all questions in your answers! Sadly, we see too many assignments which, arduously effected, nevertheless intimate nescience of elementary tenets of molecular biology. If the sentence above did not trigger an urge to open a dictionary, you are trying to guess, rather than confirm possibly important information.|
| + | <tr class="s1"><td class="l1">[[Apache]]</td></tr> |
| − | num=4|
| + | <tr class="s2"><td class="l1">[[MySQL]]</td></tr> |
| − | ord=fourth|
| + | <tr class="s1"><td class="l1">[[Tools for the bioinformatics lab]]</td></tr> |
| − | due = Monday, November 12 at 10:00 in the morning}}
| + | <tr class="s2"><td class="l1">[[GBrowse|GBrowse and LDAS]]</td></tr> |
| | + | <tr><td class="sp"> </td></tr> |
| | + | </table> |
| | | | |
| | + | ===Programming=== |
| | + | <table width="100%" > |
| | + | <tr class="s1"><td class="l1">[[IDE|IDE (Integrated Development Environment)]]</td></tr> |
| | + | <tr class="s2"><td class="l1">[[Regular Expressions]]</td></tr> |
| | + | <tr class="s1"><td class="l1">[[Screenscraping]]</td></tr> |
| | | | |
| − | <div style="padding: 5px; background: #BDC3DC; border:solid 1px #AAAAAA;"> | + | <tr class="s2"><td class="l1 mw-collapsible mw-collapsed" data-expandtext="Expand subtopics" data-collapsetext="Collapse">[[Perl]] |
| − | ==(1) Preparation== | + | <div class="mw-collapsible-content"> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl basic programming]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Perl hash example]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl LWP example]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Perl MySQL introduction]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl OBO parser]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Perl basic programming]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl programming exercises 1]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Perl programming exercises 2]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl programming Data Structures]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Perl references]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl simulation]]</td></tr></table> |
| | + | <table width="100%"><tr class="s2"><td class="l2">[[Perl: Object oriented programming]]</td></tr></table> |
| | + | <table width="100%"><tr class="s1"><td class="l2">[[Perl: Ugly programming]]</td></tr></table> |
| | </div> | | </div> |
| | + | </td></tr> |
| | | | |
| | + | <tr class="s1"><td class="l1">[[BioPerl]]</td></tr> |
| | + | <tr class="s2"><td class="l1">[[PHP]]</td></tr> |
| | + | <tr class="s1"><td class="l1">[[Data modelling]]</td></tr> |
| | + | <tr class="s2"><td class="l1">BioPython <!-- (scope, highlights, installation, use, support) --></td></tr> |
| | + | <tr class="s1"><td class="l1">Graphical output <!-- (PNG and SVG) --></td></tr> |
| | + | <tr class="s2"><td class="l1">[[Autonomous agents]]</td></tr> |
| | + | </table> |
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| − | <div style="padding: 5px; background: #E9EBF3; border:solid 1px #AAAAAA;"> | + | ===Algorithms=== |
| − | ===Template choice and sequence (1 marks)=== | + | <table width="100%" > |
| − | </div> | + | <tr class="sh"><td class="l1">Algorithms on Sequences</td></tr> |
| − | <br>
| + | <tr class="s1"><td class="l2">[[Dynamic Programming]]</td></tr> |
| − | Often more than one related structure can be found in the PDB. We have touched on principles of selecting template structures in the lecture and there is a short summary of [[Template_choice_principles|template choice principles]] on this Wiki. One can either search the PDB itself through its '''Advanced Search''' interface; for example one can search for sequence similarity with a BLAST search, or search for structural similarity by accessing structures according to their CATH or SCOP classification. But one can always also use the BLAST interface at the NCBI, since the sequences contained in PDB files are accessible as a database subsection on the BLAST menu.
| + | <tr class="s2"><td class="l2">[[Multiple Sequence Alignment]]</td></tr> |
| | + | <tr class="s1"><td class="l2">[[Genome Assembly]]</td></tr> |
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| − | <div style="padding: 5px; background: #DDDDEE;"> | + | <tr><td class="sp"> </td></tr> |
| − | *Use the NCBI BLAST interface to identify all PDB files that are clearly homologous to your target APSES domain, if you haven't already done so in Assignment 2. Document that you have searched in the correct subsection of the database by selecting "pdb" on the database options menu. For the hits you find, consider how these coordinate sets differ and which features would make each more or less suitable for your task by commenting briefly on
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| − | :*sequence similarity to your target
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| − | :*size of expected model (length of alignment)
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| − | :*presence or absence of ligands
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| − | :*experimental method and quality of the data set
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| − | Then choose the '''template''' you consider the most suitable and note why you have decided to use this template.
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| | | | |
| − | * Retrieve the most suitable template structure coordinate file from the PDB.
| + | <tr class="sh"><td class="l1">Algorithms on Structures</td></tr> |
| − | </div> | + | <tr class="s1"><td class="l2">[[Docking]]</td></tr> |
| | + | <tr class="s2"><td class="l2">Protein Structure Prediction <!-- ''ab initio'' --></td></tr> |
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| − | It is not straightforward at all how to number sequence in such a project. The "natural" numbering is to start a sequential numbering from the start-codon of the full length protein and go sequentially from there. However, this does not map well with other numbering schemes we have encountered. As you know the first residue of the APSES domain as the CDD defines it is not Residue 1 of the Mbp1 protein. The first residue of the e.g. 1MB1 FASTA file '''is''' the first residue of Mbp1 protein, but the last five residues are an artifical His tag. Is H125 of 1MB1 thus equivalent to R125 in MBP1_SACCE? The N-terminus of the Mbp1 crystal structure is disordered. The first residue in the structure is ASN 3, therefore N is the first residue in a FASTA sequence derived from the cordinate section of the PDB file (the <code>ATOM </code> records; whereas the SEQRES records start with MET ... and so on. You need to remember: a sequence number is not absolute, but derived from a particular context.
| + | <tr><td class="sp"> </td></tr> |
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| − | The homology model will be based on an alignment of target and template. Thus we have to define the target sequence. As discussed in class, PDB files have an explicit and an implied sequence and these do not necessarily have to be the same. To compare the implied and the explicit sequence for the template, you need to extract sequence information from coordinates. One way to do this is via the Web interface for [http://swift.cmbi.ru.nl/servers/html/index.html '''WhatIf'''], a crystallography and molecular modeling package that offers many useful tools for coordinate manipulation tasks.
| + | <tr class="sh"><td class="l1">Algorithms on Trees</td></tr> |
| | + | <tr class="s1"><td class="l2">Computing with trees <!-- Bayesian approaches for phylogenetic trees, tree comparison) --></td></tr> |
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| − | <div style="padding: 5px; background: #DDDDEE;"> | + | <tr><td class="sp"> </td></tr> |
| − | *Navigate to the '''Administration''' sub-menu of the [http://swift.cmbi.ru.nl/servers/html/index.html WhatIf Web server]. Follow the link to '''Make sequence file from PDB file'''. Enter the PDB-ID of your template into the form field and '''Send''' the request to the server. The server accesses the PDB file and extracts sequence information directly from the <code>ATOM </code> records of the file. The results will be returned in PIR format. Copy the results, edit them to FASTA format and save them in a text-only file. Make sure you create a valid FASTA formatted file! Use this '''implied''' sequence to check if and how it differs from the sequence ...
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| − | :*... listed in the <code>SEQRES</code> records of the coordinate file;
| + | <tr class="sh"><td class="l1">Algorithms on Networks</td></tr> |
| − | :*... given in the FASTA sequence for the template, which is provided by the PDB;
| + | <tr class="s1"><td class="l2">Network metrics <!-- (Degree distributions, Centrality metrics, other metrics on topology, small-world- vs. random-geometric controversy) --></td></tr> |
| − | :*... stored in the protein database of the NCBI.
| + | <tr class="s2"><td class="l3">[[Dijkstras Algorithm]]</td></tr> |
| − | : and record your results.
| + | <tr class="s1"><td class="l3">[[Floyd Warshall Algorithm]]</td></tr> |
| | + | </table> |
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| − | * In a table, establish how the sequence numbers in the coordinate section of your template(*) correspond to your target sequence numbering.
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| − | </div>
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| − | :(*) <small>These residue numbers are important, since they are referenced e.g. by VMD when you visualize the structure. The easiest way to list them is via the ''Sequence Viewer'' extension of VMD.</small>.
| + | ===Communication and collaboration=== |
| − | :<small>Don't do this for every residue individually but define ranges. Look at the correspondence of the first and last residue of target and template sequence and take indels into account. Establishing sequence correspondence precisely is crucially important! For example, when a publication refers to a residue by its sequence number, you have to be able to relate that number to the residue numbers of the model as well as your target sequence.</small>.
| + | <table width="100%" > |
| − | | + | <tr class="s1"><td class="l1">[[MediaWiki]]</td></tr> |
| − |
| + | <tr class="s2"><td class="l1">[[HTML essentials]]</td></tr> |
| | + | <tr class="s1"><td class="l1">[[HTML 5]]</td></tr> |
| | + | <tr class="s2"><td class="l1">[[SADI|SADI Semantic Automated Discovery and Integration]]</td></tr> |
| | + | <tr class="s1"><td class="l1">[[CGI]]</td></tr> |
| | + | <tr><td class="sp"> </td></tr> |
| | + | </table> |
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| − | <div style="padding: 5px; background: #E9EBF3; border:solid 1px #AAAAAA;"> | + | ===Statistics=== |
| − | === The input alignment (1 mark)=== | + | <table width="100%" > |
| − | </div> | + | <tr class="s1"><td class="l1">[[Pattern discovery]]</td></tr> |
| − | <br>
| + | <tr class="s2"><td class="l1">Correlation <!-- (Covariance matrices and their interpretation, application to large problems, collaborative filtering, MIC and MINE) --></td></tr> |
| | + | <tr class="s1"><td class="l1">Clustering methods <!-- (Algorithms and choice (including: hierarchical, model-based and partition clustering, graphical methods (MCL), flow based methods (RRW) and spectral methods). Implementation in R if possible) --></td></tr> |
| | + | <tr class="s2"><td class="l1">Cluster metrics <!-- (Cluster quality metrics (Akaike, BIC)–when and how) --></td></tr> |
| | + | <tr class="s1"><td class="l1">[[Map equation|The Map Equation]] </td></tr> |
| | + | <tr class="s2"><td class="l1">Machine learning <!-- (Classification problems: Neural Networks, HMMs, SVM..) --></td></tr> |
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| − | The sequence alignment between target and template is the single most important factor that determines the quality of your model.
| + | <tr class="s1"><td class="l1 mw-collapsible mw-collapsed" data-expandtext="Expand subtopics" data-collapsetext="Collapse">[[R]] |
| − | | + | <div class="mw-collapsible-content"> |
| − | No comparative modeling process will repair an incorrect alignment; it is useful to consider a homology model rather like a three-dimensional map of a sequence alignment rather than a structure in its own right. In a homology modeling project, typically the largest amount of time should be spent on preparing the best possible alignment. Even though automated servers like the SwissModel server will align sequences and select template structures for you, it would be unwise to use these only because they are convenient. You should take advantage of the much more sophisticated alignment methods available. Analysis of wrong models can't be expected to produce right results.
| + | <table width="100%"><tr class="s2"><td class="l2">R plotting</td></tr></table> |
| − | | + | <table width="100%"><tr class="s1"><td class="l2">[[R programming]]</td></tr></table> |
| − | The best possible alignment is usually constructed from a multiple sequence alignment that includes at least '''the target and template sequence''' and other related sequences as well. The additional sequences are an important aid in identifying the correct placement of insertions and deletions. Your alignment should have been carefully reviewed by you and wherever required, manually adjusted to move insertions or deletions between target and template out of the secondary structure elements of the template structure.
| + | <table width="100%"><tr class="s2"><td class="l2">R EDA</td></tr></table> |
| − | | + | <table width="100%"><tr class="s1"><td class="l2">R regression</td></tr></table> |
| − | In the case of Mbp1 genes however, all orthologues we have considered have no indels in the APSES domain regions. Evolutionary pressure on the APSES domains has selected against indels in the more than 600 million years these sequences have evolved independently in their respective species.
| + | <table width="100%"><tr class="s2"><td class="l2">R PCA</td></tr></table> |
| − | | + | <table width="100%"><tr class="s1"><td class="l2">R Clustering</td></tr></table> |
| − | Accordingly, all we need to do is to write the APSES domain sequences one under the other.
| + | <table width="100%"><tr class="s2"><td class="l2">R Classification <!-- Phrasing inquiry as a classification problem, dealing with noisy data, machine learning approaches to classification, implementation in R) --></td></tr></table> |
| − | | + | <table width="100%"><tr class="s1"><td class="l2">R hypothesis testing</td></tr></table> |
| − | <div style="padding: 5px; background: #DDDDEE;"> | + | <table width="100%"><tr class="s2"><td class="l2">[[Bioconductor]]</td></tr></table> |
| − | * Copy the FASTA formatted sequence for the APSES domain of your organism's Mbp1 orthologue from the sequences [[All_APSES_domains|defined in Assignment 3]] and save it as FASTA formatted text file. This is your '''target''' sequence. Compare this with the FASTA formatted file you have extracted from the PDB coordinate set. This is your '''template''' sequence. Then generate a multi-FASTA formatted file that contains both sequences, and '''pad''' the sequence(s) where required with hyphens as gap characters, so that target and template sequences have exactly the same length are aligned. Refer to the [[Assignment_4_fallback_data|'''Fallback data''']] if you are not sure about the format. (1 mark)
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| | </div> | | </div> |
| − | <br>
| + | </td></tr> |
| − |
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| − | <div style="padding: 5px; background: #BDC3DC; border:solid 1px #AAAAAA;"> | + | <tr><td class="sp"> </td></tr> |
| | + | </table> |
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| − | ==(2) Homology model== | + | ===Applications=== |
| − | </div> | + | <table width="100%" > |
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| + | <tr class="s1"><td class="l1">[[Data integration]] <!-- Add BioMart: Biodata integration, and data-mining of complex, related, descriptive data --></td></tr> |
| − | | + | <tr class="s2"><td class="l1">Text mining <!-- (Use cases, tasks and metrics, taggers, vocabulary mapping, Practicals: R-support, Python/Perl support, others...) --></td></tr> |
| | + | <tr class="s1"><td class="l1">[[HMMER]]</td></tr> |
| | + | <tr class="s2"><td class="l1">High-throughput sequencing</td></tr> |
| | + | <tr class="s1"><td class="l1">Functional annotation <!-- GFF --></td></tr> |
| | + | <tr class="s2"><td class="l1">Microarray analysis <!-- (... in R: differential expression and multiple testing; Loading and normalizing data, calculating differential expression, LOWESS, the question of significance, FWERs: Bonferroni and FDR; SAM and LIMMA) --></td></tr> |
| | + | <tr><td class="sp"> </td></tr> |
| | + | </table> |
| | + | </td></tr></table> |
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| − | <div style="padding: 5px; background: #E9EBF3; border:solid 1px #AAAAAA;">
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| − | === (2.1) SwissModel (1 mark)===
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| | </div> | | </div> |
| − | <br>
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| − | Access the Swissmodel server at '''http://swissmodel.expasy.org''' . Navigate to the '''Alignment Interface'''.
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| − | <br><div style="padding: 5px; background: #DDDDEE;">
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| − | *Paste your alignment for target and model into the form field. Refer to the [[Assignment_4_fallback_data|'''Fallback Data file''']] if you are not sure about the format. Make sure to select the correct option for the alignment input format on the form.
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| − | :<small>(You have to choose the correct format, and, if e.g. you choose a CLUSTAL format, you have to include a header line and a blank line. In the past we have seen problems with uploading alignments that have not been saved as "text only" and including periods i.e. "." in sequence names of CLUSTAL formatted alignments. Underscores appear to be safe.</small>
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| − | * Click '''submit alignment ''' and on the returned page define your '''target''' and '''template''' sequence. For the '''template sequence''' define the PDB ID of the coordinate file. Enter the correct Chain-ID.
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| − | :<small>Recently the PDB has undergone a "remediation" process in which archived coordinate files were altered by the database to conform to new format standards. One of the changes was to assign a chain identifier of "A" to all chains that did not previously have a chain identifier. SwissModel uses a derivative of coordinate sets from the PDB (a dataset they call ExPDB). Apparently the PDB proper and ExPDB have now gone out of synchrony; when I entered the (correct, according to PDB) chain designation "A" for 1MB1, SwissModel rejected the alignment with a nondescript error message. When I entered an underscore "_" instead, which would be the designation for a chain without explicit chain identifier, such as the pre-remidation versio of the coordinates, the alignment was accepted and processed. I have e-mailed SwissModel about the problem which may or may not be corrected while you are working on your assignments. If your template chain has the chain identifier "A" and your alignment gets rejected, enter an underscore instead.</small>
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| − | :<small>'''Enter''' the correct chain ID into the form-field even if you think it already appears there, don't simply accept the preloaded default. There is a bug in SwissModel's parser code that may cause incorrect strings to be sent to the server from that field. I have e-mailed SwissModel about the problem which may or may not be corrected while you are working on your assignments.</small>
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| − | *Click '''submit alignment''' and review the alignment on the returned page. Make sure it has been interpreted correctly by the server. The conserved residues have to be lined up and matching. Then click '''submit alignment''' again, to start the modeling process.
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| − | * The resulting page returns information about the resulting model. Save the '''model coordinates''' on your computer. Read the information on what is being returned by the server (click on the red questionmark icon). Paste the Anolea profile into your assignment.
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| − | :<small>Do not paste a screenshot of the result, but copy and paste the image from the Web-page! You do not need to submit the actual coordinate files with your assignment.</small>
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| − | (1 mark)
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| − | </div>
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| − | <br>
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| − | In case you do not wish to submit the modelling job yourself, or have insurmountable problems when using the SwissModel interface, you may access the result files from the [[Assignment_4_fallback_data|'''Fallback Data file''']]. Document the problems and note this in your assignment.
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| − | <div style="padding: 5px; background: #BDC3DC; border:solid 1px #AAAAAA;">
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| − | ==(3) Model analysis==
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| − | </div>
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| − | <div style="padding: 5px; background: #E9EBF3; border:solid 1px #AAAAAA;">
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| − | === (3.1) The PDB file (1 mark)===
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| − | </div>
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| − | <br>
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| − | Open your '''model''' coordinates in a text-editor (make sure you view the PDB file in a fixed-width font) and consider the following questions: (Alternatively, view the coordinates linked to the [[Assignment_5_fallback_data|'''Fallback Data file''']].)
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| − | <br><div style="padding: 5px; background: #DDDDEE;">
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| − | *What is the residue number of the first residue in the '''model'''? What should it be, based on the alignment? If the putative DNA binding region was reported to be residues 50-74 in the Mbp1 protein, which residues of the '''model''' correspond to that?
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| − | (1 mark)
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| − | </div>
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| − |
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| − | <!-- discuss flagging of loops - setting of B-factor to 99.0 -->
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| − | [...]
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| − | ===(3.2) first visualization (2 marks)===
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| − | </div>
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| − | In assignment 2 you have already studied the 1MB1 coordinate file and compared it to your organism's Mbp1 APSES domain, Since a homology model inherits its structural details from the '''template''', the model should look very similar to the original structure but contain the sequence of the '''target'''.
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| − | <br><div style="padding: 5px; background: #DDDDEE;">
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| − | *Save your '''model''' coordinates to your harddisk and visualize the structure in VMD. (Alternatively, copy and save the coordinates linked to the [[Assignment_4_fallback_data|'''Fallback Data file''']] to your harddisk.) Make an informative stereo view (see below), and paste it into your assignment.
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| − | * Discuss briefly which parts of the model may be unreliable and color these (if any) distinctly in your submitted image.
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| − | (2 marks)
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| − | </div>
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| − | [[Image:A5_Mbp1_subdomain.jpg|frame|none|Stereo-view of a subdomain within the 1MB1 structure that includes residues 36 to 76. The color gradient ramps from blue (36) to green (76).]]
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| − | ==(4) The DNA ligand==
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| − | </div>
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| − | <div style="padding: 5px; background: #E9EBF3; border:solid 1px #AAAAAA;">
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| − | ===(4.1) finding a similar protein-DNA complex (1 mark)===
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| − | </div>
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| − | One of the really interesting questions we can discuss with reference to our model is how sequence variation might be converted into changing DNA recognition sites, and then lead to changed cognate DNA binding sequences. But in order to address this, we would need to add a plausible model for how DNA is bound to APSES domains.
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| − | Since there is currently no software available that would accurately model such a complex from first principles, we will base a model of on homology modeling as well. This means we need to find a similar structure for which the complex structure is known. However, you may remember from the third assignment that the APSES domains in fungi seem to be a relatively small family. And there is no structure available of a protein-DNA complex. Now what?
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| − | Remember that homologous sequences can have diverged to the point where their sequence similarity is no longer recognizable, however their structure may be quite well conserved. Thus if we could find similar structures in the PDB, these might provide us with some plausible hypotheses for how DNA is bound by APSES domains. We thus need a tool similar to BLAST, but not for the purpose of sequence alignment, but for structure alignment. A kind of BLAST for structures. Very similar to using BLAST, we might not want to search with the entire protein, if all we are interested in is a subdomain that binds to DNA. Attempting to match all structural elements in addition to the ones we are actually interested in is likely to make the search less specific - we would find false positives that are similar to some irrelevant part of our structure. However, defining too small of a subdomain would also lead to a loss of specificity: in the extreme it is easy to imagine that the search for e.g. a single helix would retrieve very many hits that would be quite meaningless. The arrangement of the residues from 50 to 74 that we have already discussed in Assignment 2 suggests that the compact subdomain from 36 to 76 (see the image above) might be a useful structure to search with: it contains the residues we are interested in and enough of connected secondary structure elements to be structurally meaningful.
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| − | At the '''NCBI''', [http://www.ncbi.nlm.nih.gov/Structure/VAST/vast.shtml VAST] is provided as a search tool for structural similarity search.
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| − | At the '''EBI''' there are a number of very well designed structure analysis tools linked off the [http://www.ebi.ac.uk/Tools/structural.html '''Structural Analysis''' page]. As part of its MSD Services, [http://www.ebi.ac.uk/msd-srv/ssm/ '''MSDfold'''] provides a convenient interface for structure searches.
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| − | However we have also read previously that the APSES domains are members of a much larger superfamily, the "winged helix" DNA binding domains , of which hundreds of structures have been solved. These domains represent one branch of the tree of helix-turn-helix (HTH) DNA binding modules. (A recent review on HTH proteins is linked from the resources section at the bottom of this page). Winged Helix domains typically bind their cognate DNA with a "recognition helix" which precedes the beta hairpin and binds into the major groove; additional stabilizing interactions are provided by the edge of a beta-strand binding into the minor groove. This is good news: once we have determined that the APSES domain is actually an example of a larger group of transcription factors, we can compare our model to a structure of a protein-DNA complex. CATH does not provide information on complexes, but we can search the PDB with CATH codes in the following way:
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| − | * Access [http://cathwww.biochem.ucl.ac.uk/cgi-bin/cath/GotoCath.pl?cath=1.10.10.10 CATH domain 1.10.10.10].
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| − | * Navigate to the [http://www.pdb.org/ PDB home page] and follow the link to [http://www.pdb.org/pdb/search/advSearch.do Advanced Search]
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| − | * In the options menu for "Choose a Query Type" select Structure Features → CATH classification. A window will open that allows you to navigate down through the CATH tree. The interface is awkward because it does not display the actual CATH codes along with the class names, but you can view the class names on the CATH page linked above. Click on '''the triangle icons''' before "Mainly Alpha"→"Orthogonal Bundle"→"ARC repressor mutant, subunit A" then click on the link to "winged helix repressor DNA binding domain". As of this writing, this subquery matches 295 structures.
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| − | * Click on the (+) button behind the subquery to add an additional query. Select the option "Structure Summary"→"Moelcule / Chain type". In the option menus that pop up, select "Contains Protein → Yes", "Contains DNA → Yes""Contains RNA → Ignore". This selects files that contain Protein-DNA complexes.
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| − | * Check the box below this subquery to "Remove Similar Sequences at 90% identity" and click on "Evaluate Query". As of this writing, seventy complexes were returned.
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| − | * In the left-hand menu, under the Tabulate section, click on the "Collage" function to display icons of the structure files. This is a fast way to obtain an overview of the structures that have been returned. First of all you may notice that in fact not all of the structures are really different, despite selecting only to retrieve dissimilar sequences. This appears to be a deficiency of the algorithm. But you can also easily recognize how the recognition helix inserts into the major groove of most of the structures that were returned (at least those where the domain is not a very small part of a much larger complex). There is one exception: the structure 1DP7 shows how the human RFX1 protein binds DNA in a non-canonical way. We shall use structural superposition of your homology model and two of the winged-helix proteins to decide which mode of DNA binding seems to be more plausible for Mbp1 homologues.
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| − | <br><div style="padding: 5px; background: #DDDDEE;">
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| − | * Follow the procedure outlined above, from a CATH entry page up to viewing a Collage (or alternatively a tabular view) of the retrieved coordinate files. You can be very brief in your documentation, but do spend a bit of time to understand the key elements of the PDB's advanced search interface.
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| − | (1 mark)
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| − | </div>
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| − | <div style="padding: 5px; background: #E9EBF3; border:solid 1px #AAAAAA;">
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| − | ===(4.2) Preparation and superposition of a canonical complex (1 mark)===
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| − | </div>
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| − | The structure we shall use as a reference for the canonical binding mode is the Elk-1 transcription factor.
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| − | [[Image:A5_canonical_wHTH.jpg|frame|none|Stereo-view of the canonical DNA binding mode of the Winged Helix domain family. Shown here is the Elk-1 transcription factor - an ETS DNA binding domain - in complex with a high-affinity binding site (1DUX). Note how the "recognition helix" inserts into the major groove of the DNA molecule. The color gradient ramps from blue (34) to green (84). Note how the first helix of the "helix-turn-helix" architecture serves only to position the recognition helix and makes few interactions by itself.]]
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| − | The 1DUX file coordinate-file contains two protein domains and two B-DNA dimers in one asymmetric unit. For simplicity, let's delete the second copy.
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| − | * Access the PDB and navigate to the 1DUX structure explorer page. Download the coordinates to your computer.
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| − | * Open the coordinate file in a text-editor and delete the coordinates for chains <code>D</code>,<code>E</code> and <code>F</code>; you may also delete all <code>HETATM</code> records and the <code>MASTER</code> record. Save the file with a different name, e.g. 1DUX_monomer.pdb .
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| − | * Open VMD and load your homology model. Turn off the axes, display the model as a Tube representation in stereo, and color it by Index. Then load your edited 1DUX file, display this coordinate set in a tube representation as well, and color it by name. It is important that you can distinguish easily which is which
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| − | * You could use the Extensions→Analysis→RMSD calculator interface to superimpose the two strutcures '''IF''' you would know which residues correspond to each other. Sometimes it is useful to do exactly that: define exact correspondences between residue pairs and superimpose according to these selected pairs. For our purpose it is much simpler to use the Multiseq tool (and the structures are simple and small enough that the STAMP algortihm can define corresponding residue pairs automatically). Open the '''multiseq''' extension window, select the check-boxes next to both protein structures, and open the '''Tools→Stamp Structural Alignment''' interface.
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| − | * In the "'Stamp Alignment Options'" window, check the radio-button for ''Align the following ...'' '''Marked Structures''' and click on '''OK'''.
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| − | * In the '''Graphical Representations''' window, double-click on all "NewCartoon" representations for both molecules, to undisplay them.
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| − | * You should now see a superimposed tube model of your homology model and the 1DUX protein-DNA complex. You can explore it, display side-chains etc. and study some of the details of how a transcription factor recognizes and binds to its cognate DNA sequence. However, note that the model's side-chain orientations are not very reliable.
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| − | <br><div style="padding: 5px; background: #EEEEEE;">
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| − | * Orient and scale your superimposed structures so that their structural similarity is apparent and the recognition helix can be clearly seen inserting into the DNA major groove. Paste a copy of your image into your assignment. Remark briefly on which parts of the structure appear to superimpose best.
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| − | </div>
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| − | ===(4.2) Preparation and superposition of a non-canonical complex (1 mark)===
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| − | </div>
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| − | ===(4.3) Interpretation (1 mark)===
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| − | </div>
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| − | ==(5) Summary of Resources==
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| − | </div>
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| − | <br>
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| − | ;Links
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| − | :* [http://biochemistry.utoronto.ca/undergraduates/courses/BCH441H/restricted/Peitsch_2002_UseOfModels.pdf '''Review (PDF, restricted)''' Manuel Peitsch on Homology Modeling]
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| − | :* [http://biochemistry.utoronto.ca/undergraduates/courses/BCH441H/restricted/Aravind_2005_HTHdomains.pdf '''Review (PDF, restricted)''' Aravind ''et al.'' Helix-turn-helix domains] (background reading, not required reading)
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| − | :* [[Organism_list_2006|Assigned Organisms]]
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| − | :* [http://www.rcsb.org/pdb/file_formats/pdb/pdbguide2.2/guide2.2_frame.html '''PDB file format''']
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| − | :* [http://en.wikipedia.org/wiki/Structural_alignment Wikipedia on '''Structural Superposition'''] <small>(although the article is called "Structural Alignment")</small>
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| − | :* [[Assignment_4_fallback_data|'''Fallback Data page''']]
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| − | ;Alignments
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| − | :* [[All_Mbp1_T-COFFEE|Mbp1 proteins '''T-Coffee''' aligned (text version)]]
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| − | <div style="padding: 5px; background: #D3D8E8; border:solid 1px #AAAAAA;">
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| − | [End of assignment]
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| − | </div>
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| − | If you have any questions at all, don't hesitate to mail me at [mailto:boris.steipe@utoronto.ca boris.steipe@utoronto.ca] or post your question to the [mailto:bch441_2006@googlegroups.com Course Mailing List]
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| − | <Tasks: review location of fallback files; rewrite SwissModel interface section ...>
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