The beta bulge: a common small unit of nonrepetitive protein structure.

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Abstract
A .beta. bulge is a region between 2 consecutive .beta.-type H-bonds which includes 2 residues (positions 1 and 2) on 1 strand opposite a single residue (position x) on the other strand. Compared to regular .beta. structure, a .beta. bulge puts the usual alternation of side-chain direction out of register on 1 of the strands, introduces a slight bend in the .beta. sheet, and locally accentuates the usual right-handed strand twist. Almost all .beta. bulges are between antiparallel strands, usually between a narrow rather than a wide pair of H-bonds. The 2 commonest types are the classic .beta. bulge, with position 1 in approximately .alpha.-helical conformation, and the G1 .beta. bulge, with a required glycine at position 1 in approximately left-handed .alpha.-helical conformation. G1 bulges almost always occur in combination with a type II tight turn. The functional roles of .beta. bulges probably include compensating for the effects of a single-residue insertion or deletion within .beta. structure and providing the strong local twist required to form closed .beta. barrel structures.