Characterization of the assembly competent domain of myosin heavy chain

Abstract

Myosin is a hexameric molecule consisting of two heavy chains and two pairs of non-identical light chains. The mobile interaction of myosin thick filaments with actin thin filaments is the basis for muscle contraction. The heavy chain itself has two domains separated by a large hinge region. The globular head catalyzes ATP hydrolysis and undergoes an associated movement along actin filaments. The tail of the myosin heavy chain is a long alpha-helical coiled-coil that contains elements necessary for the formation of myosin filaments. Recently, a twenty-nine residue region in the human skeletal fast IId myosin heavy chain was shown to be necessary for the formation of the thick filament (Sohn, et al., 1997). Here, we have demonstrated the potential of this Assembly Competent Domain (ACD) to confer the ability to assemble upon an otherwise assembly incompetent rod fragment. We tested the effect of specific mutations in the ACD on assembly of bacterially expressed rod fragments. Single point mutations that alter or reverse the charge of single residues in the four highly conserved negatively charged amino acids within this region have no effect on ability to assemble. Also, we have found evidence for an additional region important in assembly that may serve as a target for the ACD. Together, these results indicate a model where the ACD is hierarchically the most important site for the initial steps of the formation of the thick filament, but other regions and/or proteins are important to determine the final register of myosin-myosin interactions.