8.2. Multiple Sequence Alignment

Multiple sequence alignment techniques are most commonly applied to protein sequences; ideally they are a statement of both evolutionary and structural similarity among the proteins encoded by each sequence in the alignment. We know that proteins with closely related functions are similar in both sequence and structure from organism to organism, and that sequence tends to change more rapidly than structure in the course of evolution. In multiple alignments generated from sequence data alone, regions that are similar in sequence are usually found to be superimposable in structure as well.

With a detailed knowledge of the biochemistry of a protein, you can create a multiple alignment by hand. This is a painstaking process, however. The challenge of automatic alignment is that it is hard to define exactly what an optimal multiple alignment is, and impossible to set a standard for a single correct multiple alignment. In theory, there is one underlying evolutionary process and one evolutionarily correct alignment to be generated from any group of sequences. However, the differences between sequences can be so great in parts of an alignment that there isn't an apparent, unique solution to be found by an alignment algorithm. Those same divergent regions are often structurally unalignable as well. Most of the insight that we derive from multiple alignments comes from analyzing the regions of similarity, not from attempting to align the very diverged regions.