ASSEMBLASES AND COUPLING PROTEINS IN THICK FILAMENT ASSEMBLY

Thick filaments are stable assemblies of myosin that are characteristi c of specific muscle types from both vertebrates and invertebrates. In general, their structure and assembly require remarkably precise dete rmination of lengths and diameters, structural differentiation and non equivalence of myosins, a high degree of inelasticity and rigidity, an d dynamic regulation of assembly and disassembly in response to both e xtracellular and intracellular signals. Directed assembly of myosin in which additional proteins function in key roles, therefore, is more l ikely to be significant than the simple self assembly of myosin into t hick filaments. The nematode Caenorhabditis elegans permits a wide spe ctrum of biochemical, genetic, molecular and structural approaches to be applied to the experimental testing of this hypothesis. Biochemical analysis of C. elegans thick filaments reveals that paramyosin, a hom ologue of the myosin rod that is the unique product of a single geneti c locus, exists as two populations which differ by post-translational modification. The major paramyosin species interacts with the two gene tically specified myosin heavy chain isoforms. The minor paramyosin sp ecies is organized within the cores of the thick filaments, where it i s associated stoichiometrically with three recently identified protein s P20, P28 and P30. These proteins have now been characterized molecul arly and contain unique, novel amino acid sequences. Structural analys is of the core shows that seven paramyosin subfilaments are crosslinke d by additional internal proteins into a highly rigid tubule. P20, P28 and P30 are proposed to couple the paramyosin subfilaments together i nto the core tubule during filament assembly. Mutants that affect para myosin assembly are being characterized for alterations in the core pr oteins. A fourth protein has been identified recently as the product o f the unc-45 gene. Computational analysis of this gene's DNA suggests that the predicted protein may exhibit protein phosphatase and chapero ne activities. Genetic analysis shows that three classes of specific u nc-45 mutant proteins differentially interact with the two myosins dur ing thick filament assembly. The unc-45 protein is proposed to be a my osin assemblase, a protein catalyst of thick filament assembly.