In vivo phosphorylation of titin and nebulin in frog skeletal muscle

Titin and nebulin are two major protein components of the sarcomere matrix in striated muscles. Purified titin and nebulin from frog (Xenopus laevis) skeletal muscle are similar in size and in amino acid composition to their mammalian or avian counterparts. Both proteins contain substantial amounts of protein-bound phosphate: about 5 to 6 per titin subunit and 8 to 9 per nebulin subunit. Injection of radioactive inorganic phosphate into the dorsal lymph sacs of Xenopus laevis resulted in the significant incorporation of radioactivity into titin and nebulin within three days of incubation. Purified titin from in vivo labeled frog gastrocnemius muscle contains one mole of radioactive phosphoserine per mole of titin subunit. These data indicate that phosphorylation of frog titin and nebulin occurs in vivo.