Trifluoperazine inhibits the MgATPase activity and in vitro motility of conventional and unconventional myosins.

Trifluoperazine, a calmodulin antagonist, has recently been shown to inhibit the MgATPase activity of scallop myosin in the absence of light chain dissociation (Patel et al. (2000) J Biol Chem 275: 4880-4888). To investigate the generality of this observation and the mechanism by which it occurs, we have examined the ability of trifluoperazine to inhibit the enzymatic properties of other conventional and unconventional myosins. We show that trifluoperazine can inhibit the actin-activated MgATPase activity of rabbit skeletal muscle myosin II heavy meromyosin (HMM), phosphorylated turkey gizzard smooth muscle myosin II HMM, phosphorylated human nonmuscle myosin IIA HMM and myosin V subfragment-1 (S1). In all cases half maximal inhibition occurred at 50-75 microM trifluoperazine while light chains (myosin II) or calmodulin (myosin V) remained associated with the heavy chains. In vitro motility of all myosins tested was completely inhibited by trifluoperazine. Chymotryptic digestion of baculovirus-expressed myosin V HMM possessing only two calmodulin binding sites yielded a minimal motor fragment with no bound calmodulin. The MgATPase of this fragment was inhibited by trifluoperazine over the same range of concentrations as the S1 fragment of myosin.