This manual is based on a previous one kindly written by Phil Jones.
Tutorial: The Distributed Annotation System (DAS) and Dasty3.
By making use of DAS you can take advantage of being able to view integrated information from multiple sources, without these sources needing to be aware of each other. You can also add your own DAS data source, perhaps privately in your own institution and then view the information served from this source in the context of features from other institutions.
This tutorial will enable you to understand how DAS functions, how you can find relevant DAS data sources and how you can view the data from these sources using different tools with their own strengths.
Example DAS Reference Server – The UniProt DAS Reference Server
This section will illustrate how the information is provided. Being able to use DAS to full advantage does not require you to understand how the DAS protocol works, however this will give you an insight into how straight forward it can be to develop your own DAS data source and an appreciation of the strengths and weaknesses of DAS.
DAS information is made available from DAS servers. A DAS server is a web application that serves data in the form of a series of XML documents that can be read and processed by DAS client software. There are several different types of XML document, each having a specific function, such as:
If features on a specific protein are being obtained from several different locations, it is very useful to ensure that one reliable, common sequence has been obtained. To achieve this, DAS separates DAS servers into two kinds:
You will now investigate an example of each kind of server.
The UniProt DAS Reference Server
The UniProt DAS reference server has the primary purpose of providing sequence information to DAS clients from the UniProt Knowledge Bank. It can be queried using UniProt accession numbers, Swiss-Prot protein Ids and also IPI (International Protein Index) accessions.
Navigate to http://www.ebi.ac.uk/das-srv/uniprot/das Here you will find a summary page describing the UniProt DAS server together with some example queries that can be made to the server.
You are now presented with a very simple XML file that contains the sequence of the protein Q14974 (Importin beta-1 subunit). This is the primary function of the UniProt DAS server. Note that the server also indicates the length of the protein as an integer and provides a version in the form of a 'hash' of the sequence. This is used to allow DAS clients to allow a check that all the DAS sources are referring to the same version of the protein sequence as the reference server.
In addition to being a reference server, the UniProt DAS server also acts as an annotation server, allowing the client to query details of features in the UniProt knowledge bank. This also incorporates InterPro features.
Take a careful look at the result of this search. You should be able to find positional features (note that the start and end coordinates of each feature are included) including Swiss-Prot annotation and InterPro domains, non-positional features such as a description of the protein (with start and end coordinates of 0) and also references relating to the protein.
As indicated above of course, the purpose of DAS is to allow you to retrieve protein information from multiple sources at the same time. An example of a completely separate but compatible DAS annotation server that is able to contribute further annotation of the same protein is given in the next task:
Here you will find additional annotation of the same protein from the MSD Motif database at the EBI. Notice that in this case the version is not the same as the version from the UniProt DAS server for the same protein accession. A typical DAS client should note this discrepancy and warn the user that the protein sequence being annotated is possibly not compatible.
This is the annotation server based at UCL in London that also provides features for the same protein. In this case the version is the same as the UniProt DAS server.
Clearly there are potentially many different DAS servers that may provide annotation that is useful for you. The next section describes how you can manually find DAS services. After this you will look at how DAS clients allow you to visualize DAS data, including handling the discrepancy described above.
Finding DAS sources – the DAS Registry Service at the Sanger Institute
“The purpose of the DAS registration service is to keep track which DAS services are around, which DAS commands they are understanding and about the coordinate systems of their data.” http://www.dasregistry.org
This valuable service provides a human web interface that allows you to search for DAS sources, test their status, examine their reliability and learn more about what they offer. At the time of writing, a total of 219 servers from 36 institutions in 14 different countries are included in the registry.
In addition to the human-searchable interface, the service can be accessed directly and transparently by DAS clients. This allows DAS clients to find relevant DAS services for you without you having to perform an exhaustive search.
Navigate to the registry at the URL at the head of this section.
Use the search box on this page to find all DAS annotation servers that use UniProt as their 'authority'.
Select any one of these and follow the information link to find out details of this server including how reliable it has been recently. You may also wish to test it now to find out if it is supporting all of the functionality that it claims to at the present time.
Dasty3 – A New Client Designed for Protein DAS Display
Dasty 3 is an example of a DAS client that is designed to display protein features in the context of the primary protein structure.
Navigate to http://www.ebi.ac.uk/dasty/
In the box labelled “Protein ID” enter the UniProt Protein Accession Q14974, and then click “GO”.
You should now see a progress bar, with messages indicating the name of the DAS server that data is currently being loaded from. Note that Dasty 3 is making direct use of the DAS registry and so you do not need to specify where the client retrieves the data from – this is automatic.
Near the top of the page, click once on “System Information” and then on “Annotation Servers Log”. This will reveal the identities of all of the DAS servers that have been queried for features of protein Q14974.
This will display a list of different DAS servers, the details of which have been obtained from the DAS registry. Note that when you searched, you didn't select a particular collection of servers which means that Dasty queried the default ones. In practice, you may well want to select the DAS servers included in your search. If you look closely at each line, you will see that the result of the search has been recorded for each DAS server. Results in green indicate a successful return. Results in red or orange indicate that either the accession number is not included in the available results for a particular DAS server, or that the server is not responding correctly.
Now click on “System Information” again to hide the DAS server list. Then Click once on the heading “Sequence” to reveal additional information about the protein. This is interactive and will be modified as you hover over or click on features on the DAS tracks below.
Try clicking on different features on the DAS tracks and observe the effect of this on the “Sequence” and “3D Structure” sections on the page. This allows you to gain a good appreciation of the sequence annotated with a particular feature, as well as the reported details of the feature.
The DAS track view includes a lot of information apart from the tracks themselves, such as the category, type and source of each feature track. Coupled with the potential for the protein to be very long and the features potentially being short or densely packed, this can make finding the specific feature you are interested in more difficult. Dasty 3 offers several ways of organising the way the data is displayed that help to solve this problem.
Changing which columns are visible: Try clicking on the different options available immediately under the heading “Manipulation Options”. You should be able to hide and show columns on the main DAS view and change the sort order of the DAS tracks.
Zooming: Try dragging the grey grab handles on the bar under the heading “Manipulation Options”. This will allow you to zoom into the details of the DAS tracks and look at a smaller portion of the protein.
Changing the order of the DAS tracks: You can grab an entire DAS track with your mouse and move it up or down in the display. This is useful if you are interested in comparing the relative position of different kinds of features, or perhaps even the same kinds of features as predicted by different sources.
As described previously, DAS annotation servers are able to serve non-positional information as well as positional features. Typically this is done by setting both the start and end coordinates of the feature to 0. Where Dasty 3 finds such features, it displays them separately.
Scroll to the bottom of the page and click on the heading “Non Positional Features” which will reveal the features. Note that this includes references in which the protein is described as well as a description of the protein.