COMPARISON OF THE EFFECTS OF CALPONIN AND A 38-KDA CALDESMON FRAGMENTON FORMATION OF THE STRONG-BINDING STATE IN GHOST MUSCLE-FIBERS

We studied the conformational changes in actin filaments induced by th e binding of calponin or a 38-kDa fragment of caldesmon, two actin-bin ding proteins known to inhibit actin-activated ATP hydrolysis by phosp horylated smooth muscle myosin. The F-actin in myosin-free muscle fibe rs (ghost fibers) was labeled with fluorescein-5-maleimide and the con formational change in actin was determined by polarized fluorimetry. D ata show that both calponin and the 38-kDa caldesmon fragment inhibit the conformational changes in F-actin that are compatible with the ''s trong-binding'' state between myosin heads and actin. Tropomyosin slig htly reduces the effect produced by calponin, but enhances the effect produced by the 38-kDa caldesmon fragment. (C) 1996 Academic Press, In c.