Synthesis of homocysteine thiolactone by methionyl-tRNA synthetase in cultured mammalian cells.

Homocysteine thiolactone is a product of an error-editing reaction, catalyzed by Escherichia coli and Saccharomyces cerevisiae methionyl-tRNA synthetases, which prevents incorporation of homocysteine into tRNA and protein both in vitro and in vivo. Here, homocysteine thiolactone is also shown to be synthesized by cultured mammalian cells such as human cervical carcinoma (HeLa), mouse renal adenocarcinoma (RAG), and Chinese hamster ovary (CHO) cells labeled with [35S]methionine, but not by normal human and mouse (Balb/c 3T3) fibroblasts. A temperature-sensitive methionyl-tRNA synthetase mutant of CHO cells, Met-1, does not make the thiolactone at the non-permissive temperature. The data indicate that methionyl-tRNA synthase is involved in synthesis of homocysteine thiolactone in CHO cells, thereby extending this important proofreading mechanism to mammalian cells.