Tools for Integrated Sequence/Structure Analysis

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Tools for Integrated Sequence/Structure Analysis

Comparisons among multiple proteins are essential for research on protein structure and function. Even when a single protein is the primary focus of a study, comparisons with a broader set of related proteins can provide valuable information on what aspects of sequence and structure are conserved. Conservation may reflect requirements for some shared aspect of function such as folding, catalysis, or binding.

Methods for linking sequence and structure data are also important for functional analyses. Many more sequences than structures are available, providing more complete information on conservation, but structural comparisons can provide information not available from sequence alone. Structural superpositions can reveal residue equivalences between proteins even when their sequences are too divergent to align with confidence. Further, different conformations of the same protein can yield insights into function.

To these ends, UCSF Chimera includes a suite of tools for:

displaying sequence alignments and automatically linking the sequences with structures

calculating conservation of sequence (entropy, variability) and structure (RMSD) and showing the values on structures

superimposing structures with or without pre-existing sequence alignments

creating alignments using sequence and/or secondary structure information

creating sequence alignments based only on spatial proximities in structural superpositions

These sequence/structure tools are not isolated but play important roles in other features, including superimposing structures shown with the structureViz Cytoscape plugin and determining residue equivalences for morphing. They also work seamlessly with other Chimera features: for example, a macromolecular complex could be modeled by fitting subunit structures into EM density, then sequence alignments could be used to evaluate whether the predicted interfaces are conserved. Although the sequence/structure tools were designed primarily for use with proteins, they also work with DNA and RNA.
The sequence/structure tools are also used by various databases and Web servers via the Chimera Web data mechanism:

The Structure-Function Linkage Database - Chimera Web data files are provided for viewing enzyme structures, active sites (Chimera sessions), and sequence alignments.
ModBase - Choosing the "launch Chimera" option to show a modeled structure displays it along with the template structure and their pairwise sequence alignment.
The ConSurf Server - Clicking the link to view results in Chimera displays the query structure and output multiple sequence alignment colored by the server's conservation measure, as shown below.

The sequence/structure tools are under continued development. These and related features are outlined below, with links to full documentation including descriptions of capabilities not mentioned in this page. See also:

Tools for integrated sequence-structure analysis with UCSF Chimera. Meng EC, Pettersen EF, Couch GS, Huang CC, Ferrin TE. BMC Bioinformatics. 2006 Jul 12;7:339.

Sequence Viewer

The Multalign Viewer tool displays individual sequences and multiple sequence alignments. Sequence alignments can be read from external files (several formats) or created by other tools in Chimera. Structures opened in Chimera are automatically associated with sufficiently similar sequences in the alignment. After association,
   • mousing over a residue in the sequence shows its structure residue number
   • selecting in the sequence selects residues in the structure(s) and vice versa
   • structures can be superimposed using the sequence alignment
Various measures of sequence conservation and structural variation (RMSD) can be computed and shown above the sequences as histograms, and on the structures with color or worm radius. Secondary structure elements can be depicted as colored boxes or regions on the alignment. Regions can also be created by hand.

Coloring by Conservation

A structure can be colored to show values of an attribute such as residue conservation. Opening a sequence alignment in Chimera automatically displays it in Multalign Viewer and associates it with any similar structures (a few residue mismatches are allowed). A variety of conservation measures can be computed. The image was created using the PFAM Carb_anhydrase seed alignment PF00194_seed.slx and includes a color key and 2D labels.

Superimposing Structures

There are several ways to superimpose structures in Chimera:

MatchMaker performs a fit after automatically identifying which residues should be paired. Pairing uses both sequence and secondary structure, allowing similar structures to be superimposed even when their sequence similarity is low to undetectable. The figure shows five distantly related proteins (pairwise sequence identities <25%) from the SCOP WD40 superfamily before and after MatchMaker superposition with default parameters.
Structures can be matched using a pre-existing sequence alignment.
The exact atoms to pair can be specified with the match command. This works on any type of structure, while the preceding methods apply only to peptide and nucleotide chains.
Structures can be superimposed manually by activating/deactivating them for motion and using the mouse.

Structure-Based Sequence Alignment

Given two or more superimposed structures, Match→Align creates a corresponding sequence alignment. The user specifies a distance cutoff for residues allowed to be in the same column of the output alignment. In proteins, the distances are measured between α-carbons. The method is independent of residue types and how the structures were superimposed.
The figure shows a superposition from MatchMaker of five proteins from the SCOP WD40 superfamily and a corresponding sequence alignment from Match→Align, automatically shown in Multalign Viewer. In the sequence alignment, light green and yellow boxes indicate strands and helices, while the headers RMSD and Conservation show spatial and sequence conservation, respectively.

Showing ConSurf Results

The ConSurf Server provides results as Chimera Web data; after browser configuration, a single click displays the color-coded query structure and multiple sequence alignment with phylogenetic tree and custom headers in a locally installed copy of Chimera (details).
Special thanks to Elana Erez and the Ben-Tal and Pupko groups at Tel Aviv University, and to Fabian Glaser at the Technion.