Smooth Muscle Myosin Light Chain Kinase (MLCK) is a multifunctional regulatory protein of smooth muscle contraction (SMC) and a key element in ligand-mediated endothelial cell gap formation and vascular permeability, motility and morphology. Smooth muscle MLCK exists in at least two isoforms, short (~150kD) and long (210kD) which are identical except for an extended amino terminus with two additional putative actin-binding motifs in the long isoform. MLCK is phosphorylated by several kinases including protein kinase A (PKA) and mediates its function by phosphorylating 20kD myosin light chain (MLC20). Phosphorylation of MLCK inhibits the actin-activated ATPase of myosin II by reducing its affinity for actin. MLCK possesses a counter-balancing role in vascular regulation, by mediating vasoconstriction via direct action on SMCs and vasodilation via action on endothelial cells (ECs).