Implicated in the level of global muscle contraction and cardiac function (By similarity). Phosphorylates a specific serine in the N-terminus of a myosin light chain. ATP + L-seryl-[myosin light chain] = ADP + H(+) + O-phospho-L-seryl-[myosin light chain] ATP + L-threonyl-[myosin light chain] = ADP + H(+) + O-phospho-L-threonyl-[myosin light chain] May interact with centrin. Colocalizes with phosphorylated myosin light chain (RLCP) at filaments of the myofibrils. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.