X-ray diffraction evidence for α-helical coiled-coils in native muscle

Abstract

An improved α-diagram has been obtained from a native molluscan “catch” muscle (the anterior byssal retractor muscle of Mytilus edulis). The most striking aspect of the diagram is a strong near-equatorial layer line at about 89 A. The 5·1 A spacing is meridional. These features are accounted for by a coiled-coil α-helical structure. Computations indicate that the best fit is obtained with a two-chain structure when various types of disorder are assumed. A possible model is described having a densely packed hydrophobic region between the chains, and most of the polar groups pointing out into the solvent. An implication of the coiled-coil structure is that the α-helix in proteins requires stabilization by side-chain interactions.