Calmodulin accelerates the rate of polymerization of human platelet actin and alters the structural characteristics of actin filaments.

Abstract

Calmodulin stimulated the rate of Mg2+-induced polymerization of human platelet actin. The stimulatory effect was due to an increase in the nucleation phase of the reaction; there was no effect on the steady-state viscosity. The calmodulin antagonist trifluoperazine blocked the stimulatory effect of calmodulin. Addition of EGTA to the reaction mixture also stimulated the rate of actin polymerization; however, the effect of calmodulin on actin polymerization is not due to Ca2+ chelation, as is presumed to be the case for EGTA. Electron microscopy revealed structural differences in the filaments prepared in the presence of calmodulin as compared to those prepared with trifluoperazine. In the presence of calmodulin, the filaments were thicker, suggesting that they consisted of multiple actin polymers. In addition, numerous projections were present perpendicular to the filaments, as well as localized areas of filament bundling. It was not possible to demonstrate a direct interaction between calmodulin and actin, which raises the possibility that the calmodulin effect may be indirect through a calmodulin-binding protein or calmodulin-dependent enzyme. Regardless of whether calmodulin is acting directly or indirectly, these results provide evidence that calmodulin may play a regulatory role in either the polymerization of actin or in determining the structural characteristics of actin filaments.

Actin-independent maturation of rabies virus in neuronal cultures

“…To reassess the involvement of actin in the maturation of the neurotropic rabies virus we have examined the effect of agents capable of producing actin filament disruption in neuronal cells. These agents function by inhibiting Ca 2+-mediated [EGTA (Birtch & Allen, 1980) and A23187 (Osborn & Weber, 1980a)] and calmodulin-operated [chlorpromazine and trifluoperazine (Osborn & Weber, 1980b;Piazza & Wallace, 1985)] mechanisms.…”

Calmodulin and wound healing in the coenocytic green alga Ernodesmis verticillata (Kützing) Børgesen

“…Therefore, it appears likely that CaM functions, at least in part, in regulating the assembly of actin into functional MF bundles. Calmodulin has been found to promote the rate of actin polymerization in human platelets (Piazza and Wallace 1985) and it is possible that it may operate in a similar manner in Ernodesmis, but such suggestions must await further work. Minimally, our results indicate that CaM is acting at some point in the transduction sequence prior to MF bundle assembly since W-7 disrupts normal bundle formation, either directly or indirectly, while W-5 does not.…”

Calmodulin and wound healing in the coenocytic green alga Ernodesmis verticillata (Kützing) Børgesen: Ultrastructure of the cortical cytoskeleton and immunogold labeling

“…Examples of known Ca 2 § dent interactions of CaM with actin-binding proteins include several proteins that cross-link actin filaments like erythrocyte adducin (Mische et al 1987), those that link actin filaments to other proteins (Anderson and Morrow 1987;Okabe and Sobue 1987), and those regulating smooth-muscle contraction including fodrin (Wang et al 1987), caldesmon (Sobue et al 1988), and myosin light-chain kinase (Yagi et al 1978). In addition, CaM is known to increase the rate of polymerization of F-actin in human platelets (Piazza and Wallace 1985) and to affect actin gelation (Kotani et al 1985;Pacaud and Molla 1987). In Ernodesmis CaM may bind to and activate a yet-unidentified protein, which in turn regulates actin-based motility in these cells.…”