Phosphorylation of threonyl- and seryl-tRNA synthetase by cAMP-dependent protein kinase. A possible role in the regulation of P1,P4-bis(5'-adenosyl)-tetraphosphate (Ap4A)

Threonyl-tRNA synthetase has been shown to be phosphorylated in reticulocytes (Dang, C.V., Tan, E.M., and Traugh, J.A., (1988) FASEB J. 2, 2376-2379). Upon incubation of reticulocytes with 8-bromo-cAMP, phosphorylation of threonyl-tRNA synthetase is stimulated approximately 2-fold, an increase similar to that observed with ribosomal protein S6. To analyze the effects of phosphorylation on activity, threonyl-tRNA synthetase has been purified to apparent homogeneity from rabbit reticulocytes utilizing a four-step purification procedure with the simultaneous purification of seryl-tRNA synthetase. Both synthetases are phosphorylated in vitro by the cAMP-dependent protein kinase. Prior to phosphorylation, the two synthetases produce significant amounts of P¹,P⁴-bis(5'-adenosyl)-tetraphosphate (Ap₄A) in the presence of the cognate amino acid and ATP, with activities comparable to that of lysyl-tRNA synthetase. Phosphorylation has no effect on aminoacylation, but an increase in Ap₄A synthesis of up to 6-fold is observed with threonyl-tRNA synthetase and 2-fold with seryl-tRNA synthetase. Thus, cAMP-mediated phosphorylation of specific aminoacyl-tRNA synthetases appears to be a potential mode of regulation of Ap₄A synthesis in mammals.