Methionine synthase is localized to the nucleus in Pichia pastoris and Candida albicans and to the cytoplasm in Saccharomyces cerevisiae [Enzymology]

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 S-adenosyl methionine. Here, we report that MS is localized to the nucleus of Pichia pastoris and Candida albicans but cytoplasmic in Saccharomyces cerevisiae. P. pastoris strain carrying a deletion of MET6 gene encoding MS (Ppmet6) exhibits methionine as well as adenine auxotrophy indicating that MS is required for methionine as well as adenine biosynthesis. Nuclear localization of P. pastoris MS (PpMS) was abrogated by the deletion of 107 C-terminal amino acids or R742A mutation. In silico analysis of PpMS structure indicated that PpMS may exist in a dimer-like configuration in which R742 of a monomer forms salt-bridge with D113 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 Ppmet6. Thus, association of two PpMS monomers through the interaction of R742 and D113 is essential for catalytic activity and nuclear localization. When PpMS is targeted to cytoplasm employing a heterologous nuclear export signal, it is expressed at very low levels and is unable to reverse methionine and adenine auxotrophy of Ppmet6. Thus, nuclear localization is essential for the stability and function of MS in P. pastoris. We conclude that nuclear localization of MS is a unique feature of respiratory yeasts such as P. pastoris and C. albicans and it may have novel moonlighting functions in the nucleus.

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