THE POSSIBLE ROLE OF MYOSIN A1 LIGHT-CHAIN IN THE WEAKENING OF ACTIN-MYOSIN INTERACTION

The effects resulting from the removal of the N-terminus of myosin Al by limited papain cleavage are investigated. The myosin and heavy mero myosin K+-ATPase and Ca2+-ATPase activities, and actin-activated ATPas e activity of heavy meromyosin (HMM) and subfragment-1, are studied. M yosin and HMM preparations devoid of the Al N-terminus exhibits lower Ca2+-ATPase activities at low ionic strength whereas no differences in K+- or Ca2+-ATPase activities are observed at high ionic strength. Di rect binding of actin to monomeric myosin under K+-activated ATPase co nditions is much more effective for myosin containing a shortened Al l ight chain. The kinetic constants K-app for actin and V-max are calcul ated from actin-activation curves for HMM and subfragment-1. The kinet ic constants for HMM are determined under conditions assuring saturati on of regulatory light chains (RLC) either with Mg2+ or Ca2+ The remov al of the Al N-terminus influences the actin-myosin interaction in a C a2+- and phosphorylation-dependent manner; in most cases, this leads t o an increase in affinity. In the case of subfragment-1, the removal o f the N-terminus of Al led to a decrease in affinity. It is reasonable to assume that the intact Al light chain may cause weakening of the a ctin-myosin interaction under certain conditions. This weakening may b e regulated by RLC phosphorylation and RLC-bound calcium-for-magnesium exchange. Such an effect requires a structural minimum that is presen t in HMM but not in subfragment-1. Implications of such a role for the Al N-terminus in the myosin-actin interaction are discussed. (C) 1997 Elsevier Science B.V.