Purification and immunological characterization of DNA polymerase-alpha from human acute lymphoblastic leukemia cells

Generation of multiple forms of methionyl-tRNA synthetase from the multi-enzyme complex of mammalian aminoacyl-tRNA synthetases by endogenous proteolysis

Methionyl-tRNA synthetase occurs free and as high-molecular-weight multi-enzyme complexes in rat liver. The free form is purified to near homogeneity by conventional column chromatography and affinity chromatography on tRNA-Sepharose. The native molecular weight of free methionyl-tRNA synthetase is 64 500, based on its sedimentation coefficient of 4.5 S and Stokes radius of 33 A. The free methionyl-tRNA synthetase apparently belongs to alpha-type subunit structure, since the subunit molecular weight is 68 000, as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Methionyl-tRNA synthetase is dissociated from the high-molecular-weight synthetase complex by controlled trypsinization, according to Kellermann, O., Viel, C. and Waller, J.P. (Eur. J. Biochem. 88 (1978) 197-204). The dissociated, free methionyl-tRNA synthetase is subsequently purified to near homogeneity. The subunit structure of dissociated methionyl-tRNA synthetase is identical to that of endogenous free methionyl-tRNA synthetase. Anti-serum raised against Mr 104 000 protein in the synthetase complex, specifically inhibited methionyl-tRNA synthetase in both the free and the high-molecular-weight forms to the same extent. These results suggest that the occurrence of multiple forms of methionyl-tRNA synthetases in mammalian cells may, in part, be due to proteolytic cleavage.