Myo1b is a widely expressed myosin-I isoform that concentrates on endosomal and ruffling membranes and is thought to play roles in membrane trafficking and dynamics. It is one of the best characterized myosin-I isoforms and appears to have unique biochemical properties tuned for tension sensing or tension maintenance. We determined the key biochemical rate constants that define the actomyo1b ATPase cycle at 37 °C and measured the temperature dependence of ATP binding, ADP release, and the transition from a nucleotide inaccessible state to a nucleotide accessible state (kalpha). The rate of ATP binding is highly temperature sensitive, with an Arrhenius activation energy 2-3 fold greater than other characterized myosins (e.g, myosin-II and myosin-V). ATP hydrolysis is fast, and phosphate release is slow and rate limiting with an actin dependence that is nearly identical to the steady-state ATPase parameters (V max and K ATPase ). ADP release is not as temperature dependent as ATP binding. The rates and temperature dependence of ADP release are similar to kalpha suggesting a similar structural change is responsible for both transitions. We calculate a duty ratio of 0.08 based on the biochemical kinetics. However, this duty ratio is likely to be highly sensitive to strain.Myosin-Is are the single-headed, low-molecular-weight members of the myosin superfamily that are proposed to link cellular membranes with the actin cytoskeleton. Myosin-I isoforms bind phosphoinositides directly (1) and function in several important cellular processes, including membrane retraction, macropinocytosis, phagocytosis, membrane trafficking, cellcell adhesion, and mechanical signal transduction (2-8).The biochemical mechanisms of long-tail and short-tail myosin-I isoforms have been studied (9-12), with myo1b being the best characterized short-tail isoform (13)(14)(15). The myo1b ATPase mechanism (Scheme 1) is notable in that (a) the maximum rate of ATP binding and population of the myo1b IQ weakly-bound states is > 30-fold slower than myosin-II, (b) nucleotide-free myo1b is in equilibrium between a state that binds nucleotide (AM′) and a state that does not bind nucleotide (AM), (c) the rate of transition between AM and AM′ (k +α ) states is similar to the rate of ADP release (k +5 ′), and (d) ADP release is slow and is accompanied by a rotation of the lever arm.We proposed that the strikingly slow actomyo1b ATPase rate constants are a property of all short-tail myosin-I isoforms (11,16). However, it has been shown recently that short-tailed Dictyostelium myosin-IE (not to be confused with vertebrate long-tail myo1e (11)) has kinetic rate constants that are substantially faster than vertebrate short-tail isoforms (12). Because of † EMO was supported by grants from the National Institutes of Health (GM57247 and AR051174). JHL was supported by a training grant from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (AR053461). *Corresponding author: E. Michael Ostap, Department of Physiology, University of Pe...