Kinase that phosphorylates MYL2 in vitro (By similarity). Increases cardiomyocyte contractility (By similarity). Required for sarcomere formation in the developing heart. 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] At 24 and 48 hours post fertilization, expressed only in the heart. Phosphorylated on serine residues. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.