Domain-structure of large monomeric alanyl-tRNA synthetase from Bombyx mori: evidence of a single catalytic domain.

Alanyl-tRNA synthetase of 115K dalton from Bombyx mori was cleaved into two fragments of 68K and 47K dalton with trypsin. The 47K fragment was active in aminoacylation of tRNA, whereas the 68K fragment inactive. The 47K and 68K fragments were located at the N- and C- terminal sides, respectively, in the intact enzyme. When the enzyme binds alanine specific tRNA, the tryptic digestion is inhibited. The Km value of the 47K fragment for tRNA was about 16-fold higher than that (1.4 microM) of the intact enzyme. The molecular activities of the 47K fragment and the intact enzyme were 2.2/sec and 16.8/sec, respectively. These results show that 1) Bombyx mori alanyl-tRNA synthetase functions in a monomeric state and 2) the C-terminal domain enhances affinity for tRNA and is responsible for full activity of aminoacylation.