INHIBITION OF 20-KDA MYOSIN LIGHT-CHAIN EXCHANGE BY MONOCLONAL-ANTIBODIES AGAINST 17-KDA MYOSIN LIGHT-CHAIN

Two anti-17,000 Da myosin light chain (LC17) monoclonal antibodies (MM 2 and MM10), which increase the actin-activated Mg2+-ATPase activity o f dephosphorylated smooth muscle myosin, inhibited the exchange of the 20,000 Da regulatory light chain of myosin (LC20). MM2, which shows h igher potency of activation of ATPase activity, inhibited the exchange more extensively than MM10, suggesting that there is a correlation be tween the activation of ATPase activity and the inhibition of the LC20 exchange. The inhibition of the exchange was observed for intact myos in and heavy meromyosin but not subfragment 1, suggesting that the hea vy chain at the head-rod junction is involved in the inhibition of LC2 0 exchange by anti-LC17 antibodies. Alternatively, the interaction bet ween the two heads of the myosin molecule may influence the inhibition of LC20 exchange. These results suggest that LC20 interacts with both LC17 and the heavy chain, and the interaction between LC20 and LC17 i s involved in the activation of actin-activated ATPase activity of smo oth muscle myosin.