Leucyl tRNA Synthetase. Two Forms of the Enzyme: Relation between Structural and Catalytic Properties

Two factors which are involved in an apparent interconversion between the two forms of leucyl-tRNA synthetase were separated. One of the factors, F1, is a protein of 200000 molecular weight and catalyzes the transformation of EII, i.e the fully active enzyme, into EI, i.e the transfer-inactive enzyme. The other factor, F2, is a peptide of about 3000 molecular weight which converts EI into an enzyme with catalytic and chromatographic prperties of EII A mild tryptic proteolysis of EII leads to modifications similar to those obseved after treatment of EII with factor F1. Moreover, after this proteolysis a peptide can be isolated, which is able to reassociate with EI. The only observed difference between native EII and the enzyme obtained by EI–F association is that the first cannot be dissociated by 8M urea or 6M guanidinium chloride, whereas the second, as well as EI, dissociates into two subunits. Results reported here suggest that Ei is a proteolyzed form of native leucyl-rRNA synthetase from which a small peptide is removed and that the enzyme is constituted from two subunitlike fragments. These fragments are in EI associated by low-energy bonds which are, in the native enzyme, completed by two peptide bonds at the ends of peptide F2. The association of this peptide with the two “subunits” is necessary for the interactions between the site of the enzyme for leucyladenylate and its tRNA receptor-site and for the transfer activity. Low-energy bonds are sufficient to keep this association which maintains a conformation and properties close to those of native enyzme.