Recognition of phylogenetic relationships from polypeptide chain fold similarities.

Structural similarities between proteins with no amino acid sequence homology either indicate a phylogenetic relationship, or they are merely the expression of a physically preferred way of folding a polypeptide chain. It is shown that one can distinguish between these alternatives by evaluating the "significance of the similarity". Such significances have been derived for comparison between chain folds containing beta-pleated sheets (Schulz and Schirmer, 1974; Richardson et al., 1976; Sternberg and Thornton, 1976). An extension of this method to comparisons between any two chain folds is outlined here.