Toward understanding actin activation of myosin ATPase: The role of myosin surface loops

To understand the complicated interplay when a traveling myosin head reaches interaction distance with two actins in a filament we looked to three myosin loops that early on exert their influences from the outside of the myosin. On these we conduct, functionally test, and interpret strategically chosen mutations at sites thought from crystallography to be a patch for binding the first of the two actins. One loop bears a hydrophobic triplet of residues, one is the so-called loop 2, and the third is the cardiomyopathy loop. So far as we know, the myosin sites that first respond are the two lysine-rich loops that produce an ionic strength-dependent weak-binding complex with actin. Subsequently, the three loops of interest bind the first actin simultaneously, and all three assist in closing the cleft in the 50-kDa domain of the myosin, a closure that results in transition from weak to strong binding and precedes rapid P-i release and motility. Mutational analysis shows that each such loop contact is distinctive in the route by which it communicates with its specific target elsewhere in myosin. The strongest contact with actin, for example, is that of the triplet-bearing loop. On the other hand, that of loop 2 (dependent on drawing close two myosin lysines and two actin aspartates) is probably responsible for opening switch 1 and uncovering the gamma-phosphate moiety of bound ATIP. Taking into account these findings, we begin to arrange in order many molecular events in muscle function.