Identification of three protein kinases which phosphorylate threonyl‐tRNA synthetase from rat liver

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Abstract
Threonyl‐tRNA synthetase is phosphorylated in Chinese hamster ovary cells labeled with 2Pi[(1984) J. Biol. Chem. 259,11160–11161]. Phosphorylation of the purified synthetase from rat liver has been examined with five different protein kinases. Three of the enzymes phosphorylate the synthetase, protease activated kinase I, the cAMP‐dependent protein kinase, and the Ca2+, phospholipid‐dependent protein kinase. Phosphorylation occurs exclusively on seryl residues. Two‐dimensional phosphopeptide maps of tryptic digests of the phosphorylated synthetase are distinct with each protein kinase. These data suggest that multiple phosphorylation of the synthetase may occur in vivo.