Particle character of aminoacyl-tRNA synthetases isolated from a postmicrosomal fraction

Multiple forms of tryptophanyl-tRNA synthetase from beef pancreas

Three different forms of tryptophanyl-tRNA synthetase can be isolated from pancreatic extracts. Structural, immunological and catalytic properties of these various forms have been compared. The native enzyme is a dimeric molecule of molecular weight 108000. Two other forms, of molecular weight 85000 and 82000, are composed of two polypeptide chains identical with the carboxyl terminal region of the native subunits. These molecules are supposed to derive from the original protein by removal, from the amino-terminal part of each subunit, of a fragment of 11000 to 13000 molecular weight. Such removal modifies the shape and the stability of the molecule and decreases its specific acitvity. The origin of the derived forms is attributed to proteolysis. In fact, limited proteolysis of purified tryptophanyl-tRNA synthetase, in its native form, by elastase, results in the formation of an active compound, similar to one of the tryptophanyl-tRNA synthetase derived forms. Furthermore, incubation with "elastolytic fractions" prepared from pancreatic extracts presenting a particularly high level of proteolytic activity produces the same cleavage in tryptophanyl-tRNA synthetase polypeptide chain.