HUMAN MYOSIN-IXB IS A MECHANOCHEMICALLY ACTIVE MOTOR AND A GAP FOR RHO

The heavy chains of the class IX myosins, rat myr5 and human myosin-IX b, contain within their tail domains a region with sequence homology t o GTPase activating proteins for the rho family of G proteins. Because low levels of myosin-IXb expression preclude purification by conventi onal means, we have employed an immunoadsorption strategy to purify my osin-IXb, enabling us to characterize the mechanochemical and rho-GTPa se activation properties of the native protein. In this report we have examined the light chain content, actin binding properties, in vitro motility and rho-GTPase activity of human myosin-IXb purified from leu kocytes. The results presented here indicate that myosin-IXb contains calmodulin as a light chain and that it binds to actin with high affin ity in both the absence and presence of ATP. Myosin-IXb is an active m otor which, like other calmodulin-containing myosins, exhibits maximal velocity of actin filaments (15 nm/second) in the absence of Ca2+. Na tive myosin-IXb exhibits GAP activity on rho. Class IS myosins may be an important link between rho and rho-dependent remodeling of the acti n cytoskeleton.