Co-evolution techniques provide a powerful way to extract structural information from the wealth of protein sequence data that we now have available. These techniques are predicated upon the notion that residues that share spatial proximity in a protein structure will mutate in a correlated fashion (co-evolve). This co-evolution signal can be inferred from a multiple sequence alignment, which tells us a bit about the evolutionary history of a particular protein family. If you want to have a better gauge at the power of co-evolution, you can refer to some of our previous posts (post1, post2).
This is more of a practical post, where I hope to illustrate an indexing problem (and how to circumvent it) that one commonly encounters when dealing with co-evolution signals.
Most of the co-evolution tools available Today output pairs of residues (i,j) that were predicted to be co-evolving from a multiple sequence alignment. One of the main applications of these techniques is to predict protein contacts, that is pairs of residues that are within a predetermined distance (quite often 8Å). Say you want to compare the precision of different co-evolution methods for a particular test set. Your test set would consist of a number of proteins for which the structure is known and for which sufficient sequence information is available for the contact prediction to be carried out. Great!
So you start with your target sequences, generate a number of contact predictions of the form (i,j) for each sequence and, for each pair, you check if the ith and jth residues are in contact (say, less than 8Å apart) on the corresponding known protein structure. If you actually carry out this test, you will obtain appalling precision for a large number of test cases. This is due to an index disparity that a friend of mine quite aptly described as “sequence hell”.
This indexing disparity occurs because there is a mismatch between the protein sequence that was used to produce the contact predictions and the sequence of residues that are represented in a protein structure. Ask a crystallographer friend if you have one, and you will find that in the process of resolving a protein’s structure experimentally, there are many reasons why residues would be missing in the final structure. More so, there are even cases where residues had to be added to enable protein expression and/or crystallisation. This implies that the protein sequence (represented by a fasta file) frequently has more (and sometimes fewer) residues than the proteins structure (represented by a PDB file). This means that if the ith and jth residues in your sequence were predicted to be in contact, that does not mean that they correspond to the ith and jth residues in order of appearance in your protein structure. So what do we do now?
A true believer in the purity and innocence of the world would assume that the SEQRES entries in your PDB file, for instance, would come to the rescue. The SEQRES describes the sequence of residues exactly as they appear on the atom coordinates of a particular PDB file. This would be a great way of mitigating the effects of added or altered residues, and would potentially mitigate the effects of residues that were not present in the construct. In other words, the sequences described by SEQRES would be a good candidate to validate whether your predicted contacts are present in the structure. They do, however, contain one limitation. SEQRES also describe any residues whose coordinates were missing in the PDB. This means that if you process the PDB sequentially and that some residues could not be resolved, the ith residue to appear on the PDB could be different to the ith residue in the SEQRES.
An even more innocent person, shielded from all the ugliness of the the universe, would simply hope that the indexing on the PDB is correct, i.e. that one can use the residue indexes presented on the “6th column” of the ATOM entries and that these would match perfectly to the (i,j) pair you obtained using your protein sequence. While, in theory, I believe this should be the case, in my experience this indexing is often incorrect and more frequently than not, will lead to errors when validating protein contacts.
My solution to the indexing problem is to parse the PDB sequentially and extract the sequence of all the residues for which coordinates are actually present. To my knowledge, this is the only true and tested way of obtaining this information. If you do that, you will be armed with a sequence and indexing that correctly represent the indexing of your PDB. From now on, I will refer to these as the PDB-sequence and PDB-sequence indexing.
All that is left is to find a correspondence (a mapping) between the sequence you used for the contact prediction and the PDB-sequence. I do that by performing a standard (global) sequence alignment using the Needleman–Wunsch algorithm. Once in possession of such an alignment, the indexes (i,j) of your original sequence can be matched to adjusted indexes (i',j') on your PDB-sequence indexing. In short, you extracted a sequential list of residues as they appeared on the PDB, aligned these to the original protein sequence, and created a new set of residue pairings of the form (i',j') which are representative of the indexing in PDB-sequence space. That means that the i’th residue to appear on the PDB was predicted to be in contact with the j’th residue to appear.
The problem becomes a little more interesting when you hope to validate the contact predictions for other proteins with known structure in the same protein family. A more robust approach is to use the sequence alignment that is created as part of the co-evolution prediction as your basis. You then identify the sequence that best represents the PDB-sequence of your homologous protein by performing N global sequence alignments (where N is the number of sequences in your MSA), one per entry of the MSA. The highest scoring alignment can then be used to map the indexing. This approach is robust enough that if your homologous PDB-sequence of interest was not present in the original MSA for whatever reason, you should still get a sensible mapping at the end (all limitations of sequence alignment considered).
One final consideration should be brought to the reader’s attention. What happens if, using the sequence as a starting point, one obtains one or more (i,j) pairs where either i or j is not resolved/present in the protein structure? For validation purposes, often these pairs are disregards. Yet, what does this co-evolutionary signal tell us about the missing residues in the structure? Are they disordered/flexible? Could the co-evolution help us identify low occupancy conformations?
I’ll leave the reader with these questions to digest. I hope this post proves helpful to those braving the seas of “sequence hell” in the near future.
