Phosphorylates a specific serine in the N-terminus of a myosin light chain. Also regulates actin-myosin interaction through a non-kinase activity (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] Interacts with SVIL. The C-terminus is deglutamylated by AGTPBP1/CCP1, AGBL1/CCP4 and AGBL4/CCP6, leading to the formation of Myosin light chain kinase, smooth muscle, deglutamylated form. The consequences of C-terminal deglutamylation are unknown (By similarity). Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.