Short Pseudo-repeat in Paramyosin

Abstract

PARAMYOSIN, a fibrous protein which has so far only been found in the muscles of invertebrates, gives an X-ray pattern with sharp 1.5 and 5.1 Å meridional and diffuse 10 Å near-equatorial reflexions and so is classified with α-keratin, myosin and fibrinogen as an α-fibrous protein. The X-ray patterns of these proteins differ, however, in the low-angle region where the pattern may be indexed on a repeat of 198 Å for α-keratin¹, 420 Å for striated muscle², 226 Å for fibrinogen³ and 725 Å for paramyosin⁴. The only one of these low-angle patterns which is clearly understood is that of paramyosin which may be interpreted in terms of either the two-dimensional net shown in Fig. 1(a) or a helix⁵. Electron micrographs have shown a two-dimensional pattern of spots⁶ similar to the lattice in Fig. 1(a) where the axial repeat is 725 Å, and this is reduced to 145 Å in axial projection (Fig. 1(b)). This communication reports a pseudo-repeat of 29 Å in axial projection which shows up when the paramyosin is treated with silver.