Trybus K M, Naroditskaya V, Sweeney H L
Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110, USA.
J Biol Chem. 1998 Jul 17;273(29):18423-8. doi: 10.1074/jbc.273.29.18423.
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.
平滑肌肌球蛋白的去磷酸化调节轻链（RLC）之间的相互作用参与维持酶的“关闭”状态。使用表达的包含骨骼肌肌球蛋白重链（HC）序列的嵌合平滑肌重酶解肌球蛋白来评估轻链结合结构域（或“颈部”）对调节的相对重要性。令人惊讶的是，具有骨骼肌RLC结合位点时调节作用仍然完好。一个具有由骨骼肌HC序列组成的整个α螺旋颈部的嵌合体显示出运动性有2倍的调节，肌动蛋白激活的ATP酶活性有近5倍的调节。当骨骼肌HC序列从头部/杆连接处延伸到SH1-SH2螺旋时，去磷酸化状态完全激活（即调节完全丧失）。因此，运动结构域附近的平滑肌特异性序列可能以优化RLC-杆-头部相互作用的方式定位调节结构域，从而在RLC去磷酸化时实现完全关闭状态。相反，一个将非常规肌球蛋白V的运动结构域与平滑肌肌球蛋白颈部和杆连接起来的嵌合体仅显示出2倍的调节。因此，平滑肌轻链结合区域和杆的存在不足以赋予肌球蛋白家族所有运动结构域完全的磷酸化依赖性调节。
