Molecular and cellular studies of tryptophanyl‐tRNA synthetase using monoclonal antibodies

Monoclonal antibodies referred to as Am1, Am2 and Am3 against highly purified bovine tryptophanyl-tRNA synthetase were prepared. Am2 antibodies inhibit the Trp-tRNA synthetase activity and interact with the active truncated enzyme forms (dimers of either 40-kDa or 51-kDa fragments) produced by limited proteolysis. Am1 and Am3 antibodies exert no effect on the Trp-tRNA synthetase activity; epitopes recognized by them are mapped close to one another and reside at the dispensable part of the Trp-tRNA synthetase molecule. Am1 cross-reacts with Trp-tRNA synthetases of eukaryotic, prokaryotic and archaebacterial species, as revealed by immunoblot analysis. A rapid two-step technique was developed for isolating electrophoretically homogeneous Trp-tRNA synthetase from Escherichia coli. The purified enzyme interacted with Am1, but not with Am2 and Am3 antibodies taken at the same concentrations. As in the case of eukaryotic Trp-tRNA synthetase, Am1 did not influence the activity of Trp-tRNA synthetase from E. coli. From the aforementioned results it follows that: (a) the conservation of part of the Trp-tRNA synthetase structure which is not directly involved in the formation of the catalytic centre of prokaryotic and eukaryotic Trp-tRNA synthetases suggests that the dispensable part of the molecule might be involved in some additional biological function(s) of Trp-tRNA synthetase besides tRNA(Trp) charging; (b) the common antigenic determinant in Trp-tRNA synthetase of eukaryotes, prokaryotes and archaebacteria indicates that this enzyme was presumably present in the common ancestor of the above organisms.