Purification of a low molecular weight form of rat liver arginyl-tRNA synthetase

Mammalian aminoacyl-tRNA synthetases are often found in several different molecular weight forms in the same extract, although the structural basis for this diversity is not clear. In this report, we describe the separation of high and low molecular weight forms of rat liver arginyl-tRNA synthetase, and the extensive purification to near homogeneity of the low molecular weight form of the enzyme, free of other aminoacyl-tRNA synthetases. The purified enzyme catalyses the incorporation into tRNA of 9,500 nmol of arginine/min/mg of protein under optimal assay conditions, corresponding to a turnover number of approximately 550 min-1. The molecular weight of the native enzyme was calculated to be approximately 59,000 based on a sedimentation coefficient of about 3.6 S and a Stokes radius of 39 A. The subunit molecular weight based on sodium dodecyl sulfate-acrylamide gel electrophoresis was also about 60,000, indicating that the enzyme is a monomer. The relation of this form of arginyl-tRNA synthetase to the high molecular weight form and its possible origin are discussed.