Seven Mammalian Aminoacyl‐tRNA Synthetases Associated within the Same Complex are Functionally Independent

Abstract

A heterotypic multienzyme complex from sheep liver containing 7 aminoacyl-tRNA synthetases specific for isoleucine, leucine, methionine, glutamine, glutamic acid, lysine and arginine was subjected to kinetic analyses. The study was undertaken to examine the possibility that association of these enzymes may impart kinetic properties which differ from those of their unassociated counterparts. The evidence obtained by 2 different approaches leads to the conclusion that the associated enzymes are functionally independent. Firstly, the kinetic constants of the methionyl-tRNA and lysyl-tRNA sythetase components of the complex do not differ significantly from those of their unassociated counterparts obtained after controlled proteolysis of the complex. Secondly, the methionyl-tRNA synthetase component of the complex displays identical kinetic constants; this is true whether assayed in the presence of [14C]methionine, ATP and highly enriched tRNAMet alone, or in the additional presence of the substrates required for unlabeled aminoacyl-tRNA formation by each of the other 6 enzymes. Similarly, the initial rates of [14C]aminoacyl-tRNA formation catalyzed by any of the 6 other enzymes were unaffected by the concomitant functioning of the other aminoacyl-tRNA synthetases. The sedimentation behavior of the aminoacyl-tRNA synthetase components of the complex under conditions prevailing in the tRNA aminoacylation assay indicates that they remain associated under these conditions. The implications of these findings on the structural organization of the enzymes within the complex are discussed.