Kinase that phosphorylates MYL2 in vitro. Promotes sarcomere formation in cardiomyocytes and increases cardiomyocyte contractility (By similarity). 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] Phosphorylated on serine residues. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.