Matching molecules in the catch mechanism.

A new structural model is advanced to account for the specialized "catch" contraction of molluscan smooth muscles. The myosin of the thick filaments of these muscles is pictured as comprising a single layer of molecules whose assembly and activity are controlled by the underlying core of paramyosin. This organization differs from that in other muscles in which the myosin is grouped into bundles. The catch state--in which the lifetime of the actin-myosin linkage is extended--would depend directly on this special organization that allows an interaction of the entire rod region of myosin with the paramyosin core. This interaction might be modulated by phosphorylation of paramyosin.