Influence of an actin-modulating protein from smooth muscle on actin-myosin interaction.

The actin-modulating protein from pig stomach smooth muscle (PSAM) which reduces the average filament length has two opposite effects on the interaction of actin with skeletal muscle myosin: (1) stimulation of both the Mg2+-ATPase activity and superprecipitation at low KCl concentrations, and (2) inhibition of these two interrelated processes at an ionic strength close to physiological. Both stimulation and inhibition were Ca2+-dependent, reflecting the requirement for Ca2+ for the interaction of the modulator with actin. With acto-subfragment-1, only inhibition of the actin-activated ATPase was observed. Possible implications of these effects for studies on the regulation of smooth muscle contraction are discussed.