One fundamental step in sequence analysis is the identification of homologous sequences, sequences related through common ancestry. There are many different ways of identifying homolog but they broadly fall into two categories: all-against-all comparisons and clustering.
The all-against-all approach aligns every sequence with every other one. This is straighforward to implement, relatively sensitive, and robust to variations in sequence lengths. The main downside of all-against-all comparisons is the quadratic computational cost with respect to the number of sequences.
In contrast, clustering works by using one representative sequence or profile per homologous family of genes (clusters), thus limiting the number of required comparisons to one per cluster. Assuming a fixed (or nearly fixed) number of clusters, the computational cost is (nearly) linear in the number of input sequences. Clustering methods however tend to miss more homologous relationships than the all-against-all.
Can the sensitivity of the all-against-all be achieved at the speed of clustering?
The OMA database—developed in our lab—currently relies upon an all-against-all. With 8,798,758 protein sequences from 1706 genomes in the latest release, this represents 38.7 trillion alignments. We could probably cope with a few thousands genomes more, but will struggle to get to the next order of magnitude with the current pipeline.
Furthermore, it is difficult to accept that as we increasingly sample the protein sequence universe, even though we know more and more about its diversity, the marginal computational cost of adding sequences goes higher, not lower.
In this project, we thus set out to try to achieve the sensitivity of the all-against-all at the speed of clustering.
Transitivity of homology
In principle, homology is a transitive relationship: if gene A is homologous to gene B, and gene B is homologous to gene C, this implies that gene A is homologous to gene C. Transitive relationships are typically a good fit for clustering.
In practice, however, things are more complicated. Homology can be difficult to ascertain for very divergent sequences. Furthermore, homology is not always transitive due to insertions, deletions, fusion, fissions, and other events that may cause inconsistencies in terms of matching residues across multiple homologs. This figure illustrates these problems and outlines the ideas we implemented to address them:
Putting together the ideas outlined in the figure above, we were pleasantly surprised to see that clustering can indeed be both sensitive and fast. We obtained 4-5x speed-ups across various datasets while recovering ~99.9% of all homologous relationships identified through all-against-all.
In comparison, general purpose clustering approaches such as kClust or UCLUST—which admittedly have not been designed to identify distant homologs effectively—only recover ~10% of all homologous relationships. They are, however, several orders of magnitude faster.
Only the beginning
The results of our proof-of-concept implementation are thus very encouraging. We have plans to follow up with a long list of refinement ideas, many of which we discuss in the manuscript. One essential refinement will be to parallelise the new approach. This is not as straightforward as with all-against-all compraisons, but we think it can be done.
Meanwhile, the serial variant is available as part of the OMA standalone package.
Wittwer, L., Piližota, I., Altenhoff, A., & Dessimoz, C. (2014). Speeding up all-against-all protein comparisons while maintaining sensitivity by considering subsequence-level homology PeerJ, 2:e607 DOI: 10.7717/peerj.607