Leucyl-tRNA and lysyl-tRNA synthetases, derived from the high-Mr complex of sheep liver, are hydrophobic proteins

The leucyl-tRNA and lysyl-tRNA synthetase components of the multienzyme complex from sheep liver were selectively dissociated by hydrophobic interaction chromatography on hexyl-agarose and purified to homogeneity. Conservation of activities during the purification required the presence of Triton X-100. The homogeneous enzymes corresponded to a monomer of Mr 129 000 and a dimer of Mr 2 × 79 000, respectively. Both were strongly adsorbed to the hydrophobic support phenyl-Sepharose, in conditions where the corresponding purified enzymes from yeast the and Escherichia coli were not bound. Moreover, like the corresponding enzymes from yeast but unlike those of prokaryotic origin, the purified leucyl-tRNA synthetases derived from the complex displayed affinity for polyanionic supports. It is shown that proteolytic conversion of lysyl-tRNA synthetase to a fully active dimer of Mr 2 × 64000, leads to loss of both the hydrophobic and the polyanion-binding properties. These results support the view that each subunit of lysyl-tRNA synthetase is composed of a major catalytic domain, similar in size to the subunit of the prokaryotic enzyme, contiguous to a chain extension which carries both cationic charges and hydrophobic residues. The implications of these findings on the structural organization of the complex are discussed in relation to its other known properties.