Modification of phenylalanyl-tRNA synthetase from baker's yeast by proteolytic cleavage and properties of the trypsin-modified enzyme.

Earlier studies have shown that native phenylalanyl-tRNA synthetase from baker's yeast contains two different kinds of subunits, alpha of molecular weight 73000 and beta of molecular weight 63000. The enzyme is an asymmetric tetramer alpha-2beta-2, which binds two moles of each ligand per mole. Incubation of the purified enzyme with trypsin results in an irreversible conversion: the alpha-subunit remains apparently unchanged but beta is rapidly degraded and yields a lighter species beta of molecular weight 41000. The trypsin-modified enzyme is an alpha-2beta-2 molecule which can still activate phenylalanine but cannot transfer it to tRNA-Phe; furthermore it does not bind tRNA-Phe but its kinetic parameters are identical to those of the native enzyme with respect to ATP and phenylalanine. Therefore the two beta subunits play a critical part in tRNA binding. Isolated alpha or beta subunits exhibit no significant activity and both types of subunit seem to be required for phenylalanine activation.