INHIBITION OF CROSS-BRIDGE BINDING TO ACTIN BY CALDESMON FRAGMENTS INSKINNED SKELETAL-MUSCLE FIBERS

Several regions within the 35-kDa COOH-terminal portion of caldesmon h ave been implicated in the ability of caldesmon to inhibit actin-activ ated myosin ATPase activity, To further define the functional regions of caldesmon, we have studied the effects of three chymotryptic fragme nts, one fragment produced by CNBr digestion and two fragments produce d by digestion with submaxillaris arginase C protease, on the relaxed stiffness and active force of rabbit psoas fibers. Each of the regions of caldesmon studied had either direct or indirect effects on single- fiber mechanics, The 35-kDa and 20-kDa fragments of caldesmon, like in tact caldesmon, were effective inhibitors of fiber stiffness, a measur e of cross-bridge attachment. The 7,3-kDa and 10-kDa fragments, which constitute the NH2 and COOH halves of the 20-kDa fragment, inhibited b oth relaxed fiber stiffness and active force production, but with a re duced efficacy compared to the 20-kDa fragment. These results suggest that several regions within the 35-kDa COOH-terminal region of caldesm on are required for optimum function of caldesmon and that function in cludes inhibition of weak cross-bridge attachment and force production .