MIMOSINE TARGETS SERINE HYDROXYMETHYLTRANSFERASE

The plant amino acid, mimosine, is an extremely effective inhibitor of DNA replication in mammalian cells (Mosca, P.J., Dijkwel, P. A. and H amlin, J. L. (1992) Mol. Cell. Biol. 12, 4375-4383). Mimosine appears to prevent the formation of replication forks at early-firing origins when delivered to mammalian approaching the G(1)/S boundary, and block s DNA replication when added to S phase cells after a lag of similar t o 2.5 h. We have shown previously that [H-3]mimosine can be specifical ly photocross-linked both in vivo and in vitro to a 50-kDa polypeptide (p50) in Chinese hamster ovary (CHO) cells. In the present study, six tryptic peptides (58 residues total) from p50 were sequenced by tande m mass spectrometry and their sequences were found to be at least 77.5 % identical and 96.5% similar to sequences in rabbit mitochondrial ser ine hydroxymethyltransferase (mSHMT). This assignment was verified by precipitating the [H-3]mimosine-p50 complex with a polyclonal antibody to rabbit cSHMT. The 50-kDa cross-linked product was almost undetecta ble in a mimosine-resistant CHO cell line and in a CHO gly(-) cell lin e that lacks mitochondrial, but not cytosolic, SHMT-activity. The gly( -) cell line is still sensitive to mimosine, suggesting that the drug may inhibit both the mitochondrial and the cytosolic forms. SHMT is in volved in the penultimate step of thymidylate biosynthesis in mammalia n cells and, as such, is a potential target for chemotherapy in the tr eatment of cancer.