Calcium Regulation of Calmodulin Binding to and Dissociation from the Myo1c Regulatory Domain

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Abstract
Myo1c is an unconventional myosin involved in cell signaling and membrane dynamics. Calcium binding to the regulatory-domain-associated calmodulin affects myo1c motor properties, but the kinetic details of this regulation are not fully understood. We performed actin gliding assays, ATPase measurements, fluorescence spectroscopy, and stopped-flow kinetics to determine the biochemical parameters that define the calmodulin-regulatory-domain interaction. We found calcium moderately increases the actin-activated ATPase activity and completely inhibits actin gliding. Addition of exogenous calmodulin in the presence of calcium fully restores the actin gliding rate. A fluorescently labeled calmodulin mutant (N111C) binds to recombinant peptides containing the myo1c IQ motifs at a diffusion-limited rate in the presence and absence of calcium. Measurements of calmodulin dissociation from the IQ motifs in the absence of calcium show that the calmodulin bound to the IQ motif adjacent to the motor domain (IQ1) has the slowest dissociation rate (0.0007 s-1), and the IQ motif adjacent to the tail domain (IQ3) has the fastest dissociation rate (0.5 s-1). When the complex is equilibrated with calcium, calmodulin dissociates most rapidly from IQ1 (60 s-1). However, this increased rate of dissociation is limited by a slow calcium-induced conformational change (3 s-1). Fluorescence anisotropy decay of fluorescently labeled N111C bound to myo1c did not depend appreciably on Ca2+. Our data suggest that the calmodulin bound to the IQ motif adjacent to the motor domain is rapidly exchangeable in the presence of calcium and is responsible for regulation of myo1c ATPase and motile activity.