pMIPs (major intrinsic proteins), also known as aquaporins, are membrane proteins that channel water and/or uncharged solutes across membranes in all kingdoms of life. Considering the enormous number of different bacteria on earth, functional information on bacterial MIPs is scarce. In the present study, six MIPs [iglpF1/i (glycerol facilitator 1)–iglpF6/i] were identified in the genome of the Gram-positive lactic acid bacterium iLactobacillus plantarum/i. Heterologous expression in iXenopus laevis/i oocytes revealed that GlpF2, GlpF3 and GlpF4 each facilitated the transmembrane diffusion of water, dihydroxyacetone and glycerol. As several lactic acid bacteria have iGlpFs/i in their lactate racemization operon (GlpF1/F4 phylogenetic group), their ability to transport this organic acid was tested. Both GlpF1 and GlpF4 facilitated the diffusion of D/L-lactic acid. Deletion of iglpF1/i and/or iglpF4/i in iLb. plantarum/i showed that both genes were involved in the racemization of lactic acid and, in addition, the double gilpF1 glpF4/i mutant showed a growth delay under conditions of mild lactic acid stress. This provides further evidence that GlpFs contribute to lactic acid metabolism in this species. This lactic acid transport capacity was shown to be conserved in the GlpF1/F4 group of iLactobacillales/i. In conclusion, we have functionally analysed the largest set of bacterial MIPs and demonstrated that the lactic acid membrane permeability of bacteria can be regulated by aquaglyceroporins./p
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