MLCK as a phosphorylation switch: from molecular structure to muscle function

Myosin light chain kinase (MLCK) serves as a central, phosphorylation-dependent molecular switch that governs myosin activation and muscle contractility. Upon binding Ca 2 ⁺/calmodulin, it phosphorylates the regulatory light chain (RLC) of myosin, inducing conformational transitions that shift myosin from an inhibited, folded state to an extended, force-generating configuration. MLCK is a ubiquitous kinase that phosphorylates both striated and non-striated myosin, albeit through distinct regulatory mechanisms. While the structural transition from inhibited to activated myosin and the contraction-initiation pathway are well established, structural details of MLCK activation and its precise interactions remain incompletely resolved due to the absence of a high-resolution structure of full-length MLCK. Recent advances in structural biology, including cryo-electron microscopy, have significantly expanded our understanding of myosin and MLCK dynamics. However, critical challenges remain, particularly in visualizing the molecular architecture of MLCK and conformational transitions underlying myosin activation. This review integrates current knowledge of the structural and mechanistic basis of myosin regulation by MLCK, emphasizing conformational switching, phosphorylation-dependent activation, and most importantly, its unique way of functioning in the striated and non-striated muscle systems. Understanding these molecular processes will provide critical insights into muscle contractility and its implications in disease states linked to myosin activation.