LOOP-I CAN MODULATE ADP AFFINITY, ATPASE ACTIVITY, AND MOTILITY OF DIFFERENT SCALLOP MYOSINS - TRANSIENT KINETIC-ANALYSIS OF S1 ISOFORMS

The striated muscle myosin of Placopecten moves actin faster in in vit ro motility assays and has a higher actin-activated ATPase turnover ra te than the myosin of the catch muscle. The heavy chain sequences of t he two PlacoS1s are almost identical except at the surface loop 1 near the nucleotide binding pocket, where the two sequences vary significa ntly. Argopecten striated muscle myosin is 96% identical to Placopecte n striated myosin, and both move actin with a similar velocity. To ide ntify the individual kinetic steps which differ between these myosins, we completed a transient kinetic characterization of the three myosin S1s. The two striated S1s have similar rates of nucleotide binding to S1 and to acto S1. The largest differences between the two are in the rate of ADP dissociation from S1 and affinity of ADP to S1, which dif fer by a factor of 2. The rates of nucleotide binding, nucleotide diss ociation and affinity to nucleotides of the two Placopecten S1s are si milar and agree within a factor of 2, In contrast, the affinity of act o S1 for ADP is nine times weaker for the striated acto S1 than for th e catch acto S1, compatible with the differences in motility of the Pl acopecten myosins. Thus the differences in ADP affinity to acto S1 and in the in vitro motility can be attributed to the differences in surf ace loop 1.