The double helix of tropomyosin

Abstract

THE tropomyosin molecule consists of two α helices, which form a coiled-coil a little over 400 Å long¹. Besides forming crystals, tropomyosin can be precipitated with divalent cations to produce needle shaped aggregates (showing a variety of patterns of cross striations in the electron microscope²) for which the detailed molecular packings have not been established. At low resolution, however, the α helices may be regarded as smooth rods. The structure of such aggregates could depend, to some extent, on considerations of closest packing of the rods. This possibility seems to have been first considered by Rudall³ in his discussion of the crystal structure of the α protein from Mantis egg case, where he showed that double helices may be close-packed to form sheets by staggering each double helix by one quarter of the pitch with respect to the next. I report here that the idea of a quarter stagger between tropomyosin molecules, in conjunction with certain electron microscope results, leads to a value for the pitch of the tropomyosin helix.