Kinetic analyses of a truncated mammalian myosin I suggest a novel isomerization event preceding nucleotide binding

MI1IQ is a complex of calmodulin and an epitope-tagged 85-kDa fragment representing the amino-terminal catalytic motor domain and the first of 6 calmodulin-binding IQ domains of the mammalian myosin I gene, rat myr-1 (130-kDa myosin I or MI130). We have determined the transient kinetic parameters that dictate the ATP hydrolysis cycle of mammalian myosin I by examining the properties of MI1IQ. Transient kinetics reveal that the affinity of MI1IQ for actin is 12 nM. The ATP-induced dissociation of actin-MI1IQ is biphasic. The fast phase is dependent upon [ATP], whereas the slow phase is not; both phases show a Ca2+ sensitivity. The fast phase is eliminated by the addition of ADP, 10 mu M being required for half-saturation of the effect in the presence of Ca2+ and 3 mu M ADP in the absence of Ca2+. The slow phase shares the same rate constant as ADP release (8 and 3 s(-1) in the presence and absence of Ca2+, respectively), but cannot be eliminated by decreasing [ADP]. We interpret these results to suggest that actin-myosin I exists in two forms in equilibrium, one of which is unable to bind nucleotide. These results also indicate that the absence of the COOH-terminal 5 calmodulin binding domains of myr-1 do not influence the kinetic properties of MI130 and that the Ca2+ sensitivity of the kinetics are in all likelihood due to Ca2+ binding to the first IQ domain.