The yeast glycerol 3-phosphatases gpp1p and gpp2p are required for glycerol biosynthesis and differentially involved in the cellular responses to osmotic, anaerobic, and oxidative stress

We have characterized the strongly homologous GPP1/RHR2 and GPP2/HOR2 genes , encoding isoforms of glycerol 3-phosphatase. Mutants lacking both GPP1 an d GPP2 are devoid of glycerol S-phosphatase activity and produce only a sma ll amount of glycerol, confirming the essential role for this enzyme in gly cerol biosynthesis. Overproduction of Gpp1p and Gpp2p did not significantly enhance glycerol production, indicating that glycerol phosphatase is not r ate-limiting for glycerol production. Previous studies have shown that expr ession of both GPP1 and GPP2 is induced under hyperosmotic stress and that induction partially depends on the HOG thigh osmolarity glycerol) pathway. We here show that expression of GPP1 is strongly decreased in strains havin g low protein kinase A activity, although it is still responsive to osmotic stress. The gpp1 Delta /gpp2 Delta double mutant is hypersensitive to high osmolarity, whereas the single mutants remain unaffected, indicating GPP1 and GPP2 substitute well for each other. Transfer to anaerobic conditions d oes not affect expression of GPP2, whereas GPP1 is transiently induced, and mutants lacking GPP1 show poor anaerobic growth. All gpp mutants show incr eased levels of glycerol 8-phosphate, which is especially pronounced when g pp1 Delta and gpp1 Delta /gpp2 Delta mutants are transferred to anaerobic c onditions. The addition of acetaldehyde, a strong oxidizer of NADH, leads t o decreased glycerol 3-phosphate levels and restored anaerobic growth of th e gpp1/Delta gpp2 Delta mutant, indicating that the anaerobic accumulation of NADH causes glycerol 3-phosphate to reach growth-inhibiting levels. We a lso found the gpp1 Delta /gpp2 Delta mutant is hypersensitive to the supero xide anion generator, paraquat, Consistent with a role for glycerol 3-phosp hatase in protection against oxidative stress, expression of GPP2 is induce d in the presence of paraquat,This induction was only marginally affected b y the general stress-response transcriptional factors Msn2p/4p or protein k inase A activity. We conclude that glycerol metabolism plays multiple roles in yeast adaptation to altered growth conditions, explaining the complex r egulation of glycerol biosynthesis genes.