Paramyosin and the filaments of molluscan “catch” muscles: I. Paramyosin: Structure and assembly

Abstract

Aggregates of paramyosin precipitated with divalent cations show a variety of forms with simple staining patterns and a period of 725 Å in the electron microscope. All the forms may be generated from a basic polar array where the molecules do not bond end-to-end: dark staining regions represent “gap” areas where stain can penetrate the paracrystal; light regions are “overlap” areas where stain is largely excluded. The various polar and dihedral arrays can be accounted for by a superposition of two of the basic polar arrays. This interpretation is supported by the appearance of “corrugation” at the lateral edge of the overlap regions of the fibers, by positive staining experiments, and by the “fringe” length observed at the ends of tactoids. The molecular length deduced from the band patterns is about 1275 Å, and is slightly different in paramyosins from different sources. Since the same basic arrays are found in paramyosins from different sources, however, there are similar specific interaction sites on these molecules. The polarity of the molecule implies a special amino acid sequence which gives rise to a complementary bonding pattern in a structure made of two similar chains running in the same direction. Although the three-dimensional packing of molecules cannot be inferred, these results provide new information on the local packing relations of the molecules: 2-fold screw axes relate molecules—at least locally—in neighboring rows in a paramyosin filament. Examination of the native paramyosin filaments with the knowledge of the in vitro polymorphic forms allows the recognition of interaction patterns which were not otherwise obvious. The “nodes” of the native net patterns can be interpreted as the dark staining gaps of the polar form, and the native net pattern can be generated by a regular shift of groups of molecules or subfilaments in the basic polar array. This specific stagger is commonly found in the in vitro paramyosin aggregates. A synthetic paracrystalline form has been produced having a net array with the same symmetry and axial period as the native filament. The tendency of paramyosin to form both polar and bipolar arrays has led also to the prediction of bipolarity in the native filament.