Loop 1 of transducer region in mammalian class I myosin, Myo1b, modulates actin affinity, ATPase activity, and nucleotide access

Loop 1, a flexible surface loop in the myosin motor domain, comprises in part the transducer region that lies near the nucleotide-binding site and is proposed from structural studies to be responsible for the kinetic tuning of product release following ATP hydrolysis (1). Biochemical studies have shown that loop 1 affects the affinity of actin-myosin-II for ADP, motility and the V(max) of the actin-activated Mg2+-ATPase activity, possibly through P(i) release (2-8). To test the influence of loop 1 on the mammalian class I myosin, Myo1b, chimeric molecules in which (i) loop 1 of a truncated form of Myo1b, Myo1b1IQ, was replaced with either loop 1 from other myosins; (ii) loop 1 was replaced with glycine; or (iii) some amino acids in the loop were substituted with alanine and were expressed in baculovirus, and their interactions with actin and nucleotide were evaluated. The steady-state actin-activated ATPase activity; rate of ATP-induced dissociation of actin from Myo1b1IQ; rate of ADP release from actin-Myo1b1IQ; and the affinity of actin for Myo1b1IQ and Myo1b1IQ.ADP differed in the chimeras versus wild type, indicating that loop 1 has a much wider range of effects on the coupling between actin and nucleotide binding events than previously thought. In particular, the biphasic ATP-induced dissociation of actin from actin-Myo1b1IQ was significantly altered in the chimeras. This provided evidence that loop 1 contributes to the accessibility of the nucleotide pocket and is involved in the integration of information from the actin-, nucleotide-, gamma-P(i)-, and calmodulin-binding sites and predicts that loop 1 modulates the load dependence of the motor.