Ca²⁺-dependent regulation of the motor activity of myosin V

Mouse myosin V constructs were produced that consisted of the myosin motor domain plus either one IQ motif (M5IQ1), two IQ motifs (M5IQ2), a complete set of six IQ motifs (SHM5), or the complete IQ motifs plus the coiled-coil domain (thus permitting formation of a double-headed structure, DHM5) and expressed in Sf9 cells. The actin-activated ATPase activity of all constructs except M5IQ1 was inhibited above pCa 5, but this inhibition was completely reversed by addition of exogenous calmodulin. At the same Ca²⁺ concentration, 2 mol of calmodulin from SHM5 and DHM5 or 1 mol of calmodulin from M5IQ2 were dissociated, suggesting that the inhibition of the ATPase activity is due to dissociation of calmodulin from the heavy chain. However, the motility activity of DHM5 and M5IQ2 was completely inhibited at pCa 6, where no dissociation of calmodulin was detected. Inhibition of the motility activity was not reversed by the addition of exogenous calmodulin. These results indicate that inhibition of the motility is due to conformational changes of calmodulin upon the Ca²⁺ binding to the high affinity site but is not due to dissociation of calmodulin from the heavy chain.