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J Biol Chem, Vol. 273, Issue 29, 18423-18428, July 17, 1998

The Light Chain-binding Domain of the Smooth Muscle Myosin Heavy Chain Is Not the Only Determinant of Regulation

Kathleen M. TrybusDagger , Vera NaroditskayaDagger , and H. Lee Sweeney

From the Dagger  Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110 and the  Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6085

Interactions between the dephosphorylated regulatory light chains (RLCs) of smooth muscle myosin are involved in maintaining the enzymatically "off" state. Expressed chimeric smooth muscle heavy meromyosins containing skeletal muscle myosin heavy chain (HC) sequences were used to assess the relative importance of the light chain-binding domain (or "neck") to regulation. Surprisingly, regulation remained intact with a skeletal RLC-binding site. A chimera with the entire alpha -helical neck composed of skeletal HC sequence showed 2-fold regulation of motility and nearly 5-fold regulation of actin-activated ATPase activity. Complete activation of the dephosphorylated state (i.e. complete loss of regulation) occurred when skeletal HC sequence extended from the head/rod junction to the SH1-SH2 helix. Smooth muscle-specific sequences near the motor domain may therefore position the regulatory domain in a way that optimizes RLC-rod-head interactions, thus enabling a completely off state when the RLC is dephosphorylated. Conversely, a chimera that joins the motor domain from unconventional myosin V to the smooth muscle myosin neck and rod showed only 2-fold regulation. The presence of the smooth muscle light chain-binding region and rod is therefore not sufficient to confer complete phosphorylation-dependent regulation upon all motor domains of the myosin family.

Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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