Myositis autoantibody reactivity and catalytic function of threonyl-tRNA synthetase.

Spontaneously occurring autoantibody to threonyl-tRNA synthetase found in the serum of patients with polymyositis and experimentally induced antibody against highly purified rabbit reticulocyte threonyl-tRNA synthetase were used to analyze the epitopes of threonyl-tRNA synthetase. The PL-7 autoantibody reacted with the native but not the denatured form of threonyl-tRNA synthetase, whereas the experimentally induced antibody recognized both the native and denatured forms of the enzyme. In addition, the PL-7 autoantibody specifically inhibited threonyl-tRNA synthetase activity whereas the experimentally induced antibody had no effect on aminoacylation. Thus, the epitopes recognized by the PL-7 autoantibody are formed by the tertiary structure of the enzyme and are associated with the catalytic site of the synthetase whereas the experimentally induced antibody recognizes epitopes formed by primary sequences not related to the catalytic function of the synthetase.