THE LIGHT CHAIN-BINDING DOMAIN OF THE SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN IS NOT THE ONLY DETERMINANT OF REGULATION

Interactions between the dephosphorylated regulatory light chains (RLC s) of smooth muscle myosin are involved in maintaining the enzymatical ly ''off'' state. Expressed chimeric smooth muscle heavy meromyosins c ontaining skeletal muscle myosin heavy chain (HC) sequences were used to assess the relative importance of the light chain-binding domain (o r ''neck'') to regulation. Surprisingly, regulation remained intact wi th a skeletal RLC-binding site. A chimera with the entire cu-helical n eck composed of skeletal HC sequence showed a-fold regulation of motil ity and nearly 5-fold regulation of actin-activated ATPase activity. C omplete activation of the dephosphorylated state (i.e. complete loss o f regulation) occurred when skeletal HC sequence extended from the hea d/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 comple tely off state when the RLC is dephosphorylated. Conversely, a chimera that joins the motor domain from unconventional myosin V to the smoot h muscle myosin neck and rod showed only a-fold regulation. The presen ce of the smooth muscle light chain-binding region and rod is therefor e not sufficient to confer complete phosphorylation-dependent regulati on upon all motor domains of the myosin family.