Conformational states of pig brain myosin

The conventional, longtailed myosins expressed in smooth muscle and nonmuscle tissue are generally thought to exist in two discrete monomeric configurations which in vitro can be interconverted by changes in the ionic strength of the buffer. For nonmuscle myosin, only two species had been investigated so far. Here, we show that dephosphorylated pig brain myosin, consisting of at least two electrophoretic variants, can also adopt an extended and a folded configuration as seen in rotary shadowed molecules. Monoclonal antibodies were employed to analyze the possible role of specific tail domains in these changes. One of these antibodies (a-PBM9) which binds to an epitope in the first third of the tail, was found to induce and stabilize the extended form, as seen in ELISA with soluble myosin and in rotary-shadowed immune complexes, while another antibody (a-PBM4), binding close to the carboxy terminus of the tail, showed no such effect. Our data support the model that all nonmuscle myosins can shuttle between an extended, assembly-competent and a folded or collapsed form, and that specific regions within the tail play a crucial role in this interconversion.