Macromolecular Complex of Aminoacyl‐tRNA Synthetases from Sheep Liver

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Abstract
Both the tRNA aminoacylation and amino-acid-dependent ATP-PPi exchange activities of monomeric trypsin-modified methionyl-tRNA synthetase from sheep liver are lost upon incubation with oxidized initiator tRNAMet. The inactivation, which reflects the formation of a Schiff''s base between the 5''-terminal adenosine of tRNA and a lysine within the catalytic site of the enzyme, is accompanied by the covalent attachment of 1 tRNA molecule/enzyme molecule. The affinity labeling method is applied to the sheep liver complex of MW 106 carrying 7 aminoacyl-tRNA synthetase activities, from which the monomeric trypsin-modified methionyl-tRNA synthetase (MW 68000) was derived. Upon incubation with oxidized initiator tRNAMet, the methionyl-tRNA synthetase activity of the complex is lost. Of the 11 polypeptide chains composing the high-molecular-weight complex, only 1 polypeptide chain with MW 103,000 reacts with the modified tRNAMet. The blocking by periodate-treated tRNA of the methionyl-tRNA synthetase activity in the complex has no effect on the other aminoacyl-tRNA synthetase activities. This strongly argues in favor of the independent parallel functioning of the 7 aminoacyl-tRNA synthetases associated in a high-molecular-weight complex.