We have re-examined the Ca(++)-dependent interaction of an intestinal microvillar 95- kdalton protein (MV-95K) and actin using the isolated acrosomal process bundles from limulus sperm. Making use of the processes as nuclei for assembling actin filaments, we quantitatively and qualitatively examined MV-95K's effect on filament assembly and on F- actin, both in the presence and in the absence of Ca(++). The acrosomal processes are particularly advantageous for this approach because they nucleate large numbers of filaments, they are extremely stable, and their morphology can be used to determine the polarity of any nucleated filaments. When filament nucleation was initiated in the presence of MV-95K and the absence of Ca(++), there was biased filament assembly from the bundle ends. The calculated elongation rates from both the barbed and pointed filament ends were virtually indistinguishable from control preparations. In the presence of Ca(++), MV-95K completely inhibited filament assembly from the barbed filament end without affecting the initial rate of assembly from the pointed filament end. The inhibition of assembly results from MV-95K binding to and capping the barbed filament end, thereby preventing monomer addition. This indicates that, while MV-95K is a potent nucleator of actin assembly, it is also a potent inhibitor of actin filament elongation. To examine the effects of MV-95K on F-actin in the presence of Ca(++), we developed an assay where MV-95K is added to filaments previously assembled from acrosomal processes without causing filament breakage during mixing. These results clearly demonstrated that rapid filament shortening by MV-95K results through a mechanism of disrupting intrafilament monomer-monomer interactions. Finally, we show that tropomyosin-containing actin filaments are insensitive to cutting, but not to capping, by MV-95K in the presence of Ca(++).

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