Methionine synthase is localized to the nucleus in and and to the cytoplasm in .

Methionine synthase (MS) catalyzes methylation of homocysteine, the last step in the biosynthesis of methionine, which is essential for the regeneration of tetrahydrofolate and biosynthesis of -adenosylmethionine. Here, we report that MS is localized to the nucleus of and but is cytoplasmic in The strain carrying a deletion of the gene encoding MS () exhibits methionine as well as adenine auxotrophy indicating that MS is required for methionine as well as adenine biosynthesis. Nuclear localization of MS (PpMS) was abrogated by the deletion of 107 C-terminal amino acids or the R742A mutation. analysis of the PpMS structure indicated that PpMS may exist in a dimer-like configuration in which Arg-742 of a monomer forms a salt bridge with Asp-113 of another monomer. Biochemical studies indicate that R742A as well as D113R mutations abrogate nuclear localization of PpMS and its ability to reverse methionine auxotrophy of Thus, association of two PpMS monomers through the interaction of Arg-742 and Asp-113 is essential for catalytic activity and nuclear localization. When PpMS is targeted to the cytoplasm employing a heterologous nuclear export signal, it is expressed at very low levels and is unable to reverse methionine and adenine auxotrophy of Thus, nuclear localization is essential for the stability and function of MS in We conclude that nuclear localization of MS is a unique feature of respiratory yeasts such as and and it may have novel moonlighting functions in the nucleus.