A database search employing a novel algorithm for protein structure comparison by alignment of distance matrices has revealed a striking resemblance between the tertiary structures of the bacterial toxin colicin A and globins. The globin-like domain in colicin A contains all elements essential for the toxin's lethal ionophoric activity. The structural similarity between colicin A and globins is comparable to that between globins and phycocyanins. This suggests that these three protein families, which have unrelated sequences and different functional contexts, are an example of physical convergence to a stable folding motif, the three-on-three helical sandwich.
