The contractile and enzymatic activities of myosin VI are regulated by calcium binding to associated calmodulin (CaM) light chains. We have used transient phosphorescence anisotropy to monitor the microsecond rotational dynamics of erythrosin-iodoacetamide-labeled actin with strongly bound myosin VI (MVI) and to evaluate the effect of MVI-bound CaM light chain on actin filament dynamics. MVI binding lowers the amplitude but accelerates actin filament microsecond dynamics in a Ca2+- and CaM-dependent manner, as indicated from an increase in the final anisotropy and a decrease in the correlation time of transient phosphorescence anisotropy decays. MVI with bound apo-CaM or Ca2+-CaM weakly affects actin filament microsecond dynamics, relative to other myosins (e.g., muscle myosin II and myosin Va). CaM dissociation from bound MVI damps filament rotational dynamics (i.e., increases the torsional rigidity), such that the perturbation is comparable to that induced by other characterized myosins. Analysis of individual actin filament shape fluctuations imaged by fluorescence microscopy reveals a correlated effect on filament bending mechanics. These data support a model in which Ca 2+-dependent CaM binding to the IQ domain of MVI is linked to an allosteric reorganization of the actin binding site(s), which alters the structural dynamics and the mechanical rigidity of actin filaments. Such modulation of filament dynamics may contribute to the Ca2 +- and CaM-dependent regulation of myosin VI motility and ATP utilization.
Bibliographical noteFunding Information:
Phosphorescence experiments were carried out in the Biophysical Spectroscopy Facility, University of Minnesota. The authors thank Octavian Cornea for assistance with preparation of the manuscript. This work was supported by grants from the National Institutes of Health to D.D.T. ( AR32961 , AG26160 ) and to E.M.D.L.C. ( GM071688 , GM071688-S1 , and GM097348 ). E.M.D.L.C. is an American Heart Association Established Investigator ( 0940075N ), a National Science Foundation CAREER Award recipient ( MCB-0546353 ), and a Hellman Family Fellow. B.R.M. was supported by American Heart Association predoctoral award 09PRE2230014 . H.F.C. was supported by National Institutes of Health predoctoral fellowship F31 DC009143 and in part by grants from the American Heart Association ( 0655849T ) and Yale Institute for Nanoscience and Quantum Engineering to E.M.D.L.C.
- myosin VI
- phosphorescence spectroscopy