Electron microscopic analysis of tropomyosin paracrystals

Negatively stained images of divalent cation-induced tropomyosin paracrystals show polymorphic patterns which are almost bipolar. Although these bipolar forms are naturally due to antiparallel arrays of molecules, the precise molecular arrangements have not been clarified yet except in the case of one type of these polymorphic paracrystals by Stewart and McLachlan [(1976) J. Mol. Biol. 103, 251--269]. In the previous paper we showed that the lead-induced polar paracrystal is a parallel and in-register array of tropomyosin molecules. Moreover, we have made it possible to locate a given residue on the staining pattern. By overlapping two photographic transparencies of the polar paracrystal antiparallel, directly observed images of polymorphic bipolar paracrystals could be synthesized photographically with fidelity. The overlap length between N-terminals of antiparallel pairs of molecules could be easily determined without any assumptions. Next, we considered the stabilizing forces involved in the morphogenesis of such polymorphic paracrystals. The cation-bridged attractive forces already proposed by some groups were insufficient to account for the stability of some specific forms of tropomyosin paracrystals. From the primary amino acid sequence of tropomyosin, we calculated the changes of repulsive forces between the basic residues with changes of molecular overlap length between the N-terminals of antiparallel pairs. By setting the values of charge appropriately, we could account well for the stability of the polymorphic structures observed by electron microscopy.