Protein repeats: Structures, functions, and evolution

Internal repetition within proteins has been a successful strategem on mult iple separate occasions throughout evolution. Such protein repeats possess regular secondary structures and form multirepeat assemblies in three dimen sions of diverse sizes and functions. In general, however, internal repetit ion affords a protein enhanced evolutionary prospects due to an enlargement of its available binding surface area. Constraints on sequence conservatio n appear to be relatively lax, due to binding functions ensuing from multip le, rather than, single repeats. Considerable sequence divergence as well a s the short lengths of sequence repeats mean that repeat detection can be a particularly arduous task. We also consider the conundrum of how multiple repeats, which show strong structural and functional interdependencies, eve r evolved from a single repeat ancestor. In this review, we illustrate each of these points by referring to six prolific repeat types (repeats in beta -propellers and beta -trefoils and tetratricopeptide, ankyrin, armadillo/B EAT, and leucine-rich repeats) and in other less-prolific but nonetheless i nteresting repeats. (C) 2001 Academic Press.