Kinase that phosphorylates MYL2 in vitro (PubMed:18202317, PubMed:17885681). Has been proposed to be calmodulin-dependent (PubMed:17885681), although MYL2 phosphorylation has also been observed in the presence or absence of calmodulin (PubMed:18202317). 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] Restricted to cardiomyocytes (at protein level). Down-regulated in heart after experimental myocardial infarction at the protein level; no significant changes at the mRNA level. Up-regulated in the heart from 10.5 dpc to neonates and further increased in adults. Down-regulated in aged hearts (at protein level). Phosphorylated on serine residues. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.