Mediates glycerol diffusion across the cytoplasmic membrane via a pore-type mechanism (PubMed:6998951, PubMed:7512955, PubMed:11039922, PubMed:11226336). Is highly permeable to glycerol, but less well permeated by water (PubMed:11226336). Does not transport ions (PubMed:7512955). It may also have limited permeability to various other substrates, including xylitol, erythritol, D-arabitol, L-arabitol, ribitol, D-galactitol, D-mannitol, D-sorbitol, urea, glycine, D/L-glyceraldehyde and the trivalent inorganic forms of arsenic and antimony (PubMed:6998951, PubMed:9150238, PubMed:14970228). glycerol(in) = glycerol(out) Transport of glycerol is blocked by mercuric ions (Hg(2+)) but not N-ethylmaleimide. Homotetramer (PubMed:11039922, PubMed:11226336). Exists in multiple oligomeric states (PubMed:11226336). Is predominantly disassociated in the less dense fractions (PubMed:11226336). Tetramers are stabilized while in the membrane and during affinity purification (PubMed:11226336). Expression is regulated by the HTH-type transcriptional regulator GlpR and by the cAMP-cAMP receptor protein (CRP) complex. Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Mutant grows on glucose but not on glycerol (PubMed:9150238). Disruption of the gene greatly reduces the level of uptake of both arsenite and antimonite (PubMed:9150238, PubMed:14970228). The remarkable property of effective water conductance combined with a strict exclusion of all ions including protons is mediated by two conserved asparagines which force a central water molecule to serve strictly as a hydrogen bond donor to its neighboring water molecules. Assisted by the electrostatic potential generated by two half-membrane spanning loops, this dictates opposite orientations of water molecules in the two halves of the channel, and thus prevents the formation of a 'proton wire', while permitting rapid water diffusion. Belongs to the MIP/aquaporin (TC 1.A.8) family.