Implicated in the level of global muscle contraction and cardiac function. 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. Heart and skeletal muscles. Increased expression in the apical tissue compared to the interventricular septal tissue. The disease is caused by variants affecting the gene represented in this entry. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.