Serine/arginine-rich protein-specific kinase which specifically phosphorylates its substrates at serine residues located in regions rich in arginine/serine dipeptides, known as RS domains. Phosphorylates the SR splicing factor SRSF1 and the lamin-B receptor (LBR) in vitro. Required for normal muscle development. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Exclusively expressed in skeletal and heart muscle. Expressed from embryogenesis to adulthood. Mice are viable to adulthood but display defects in skeletal muscle growth including reduced muscle mass, marked increase in centrally placed nuclei and disorganized intermyofibrillar network. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family.