The α aneurism: A structural motif revealed in an insertion mutant of staphylococcal nuclease

The x-ray crystal structure of a mutant of staphylococcal nuclease that contains a single glycine residue inserted in the C-terminal α-helix has been solved to 1.67 Å resolution and refined to a crystallographic R value of 0.170. This inserted glycine residue is accommodated in the α-helix by formation of a previously uncharacterized bulge, which we term the α aneurism. A conformational search of known protein structures has identified the α aneurism in a number of protein families, including the histocompatibility antigens and hemoglobins.