Specific aquaporins increase the ammonia tolerance of a Saccharomyces cerevisiae mep1-3fps1 deletion strain

Abstract

Aquaporins (AQPs) are channel proteins which facilitate the bidirectional membrane permeation of small neutral molecules such as water and glycerol. A convenient way to characterize their permeability is by growth of transformed Saccharomyces cerevisiae deletion strains on nutrient-limited substrates. We selected a yeast strain deficient in its endogenous ammonium transporters Mep1-3 and aquaglyceroporin Fps1 in order to study the ammonia permeability of heterologously expressed AQPs. Surprisingly, AQP-expression improved yeast growth at high, not low, concentrations of unprotonated ammonia. At neutral or mildly alkaline pH, ammonia concentrations above 10 μM decreased the growth rate and especially the number of yeast cell duplications, but did not affect the lag phase. AQP-expression raised the threshold to about 100 μM. The exchange of ammonium ions for amino acids or urea did not completely abolish this effect. AQPs capable of rescuing growth had a selectivity filter wide enough to permit passage of molecules larger than water but smaller than glycerol. It appears that the endogenous aquaglyceroporin Fps1 may, under alkaline conditions, be beneficial to yeast by facilitating the membrane permeation of an as yet unidentified molecule other than glycerol.

Keywords::

• Aquaporin
• ammonia
• Saccharomyces cerevisiae

Acknowledgements

This work was funded by the European Commission (FP7-HEALTH 201924 EDICT). We wish to thank R. Bill for suggesting the use of an automated microplate incubator, S. Hohmann for kindly providing AQP9-encoding cDNA, and J. von Bülow for helpful comments on the draft.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.