PHOSPHORYLATION OF TRYPTOPHANYL-TRANSFER RNA-SYNTHETASE BY CASEIN KINASE-II

Highly purified tryptophanyl-tRNA synthetase (E.C. 6.1.1.2) from bovine pancreas with incubated with [gamma-P-32]nucleoside triphosphates and highly purified casein kinase I or II from rabbit liver; the phosphorylation products were analyzed by means of two-dimensional gel electrophoresis. Tryptophanyl-tRNA synthetase was phosphorylated in vitro only be casein kinase II (E.C. 2.7.1.37). ATP and GTP were equally effective as phosphate donors, and serine residues of the synthetase molecules were modified exclusively. The maximal degree of modification averaged 0.15 mole phosphate per mole synthetase polypeptide chain (mol. wt. 60 kDa). Phosphorylation occurred predominantly at the most acidic isoform (pI 4.9) of the synthetase polypeptide chain. Incubation of the synthetase with alkaline or acid phosphatases did not alter its modification by casein kinase II. Possible reasons for the limited and selective modification of one form of the synthetase polypeptide chain are discussed.