Striated muscle preferentially expressed genes α and β are two serine/threonine protein kinases derived from the same gene as the aortic preferentially expressed gene-1

Aortic preferentially expressed gene (APEG)-1 is a 1.4-kilobase pair (kb) mRNA expressed in vascular smooth muscle cells and is down-regulated by vascular injury. An APEG-1 5'-end cDNA probe identified three additional isoforms. The 9-kb striated preferentially expressed gene (SPEG)alpha and the 11-kb SPEG beta were found in skeletal muscle and heart. The 4-kb brain preferentially expressed gene was detected in the brain and aorta. We report here cloning of the Il-kb SPEG beta cDNA SPEG beta encodes a 355-kDa protein that contains two serine/threonine kinase domains and is homologous to proteins of the myosin light chain kinase family. At least one Kinase domain is active and capable of autophosphorylation. In the genome, all four isoforms share the middle three of the five exons of APEG-1, and they differ from each other by using different 5'- and S'-ends and alternative splicing. We show that the expression of SPEG alpha and SPEG beta is developmentally regulated in the striated muscle during C2C12 myoblast to myotube differentiation in vitro and cardiomyocyte maturation in vivo. This developmental regulation suggests that both SPEG alpha and SPEG beta can serve as sensitive markers for striated muscle differentiation and that they may be important for adult striated muscle function.