Mediates glycerol diffusion across the cytoplasmic membrane via a pore-type mechanism. glycerol(in) = glycerol(out) Homotetramer. Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Belongs to the MIP/aquaporin (TC 1.A.8) family. All strains have an amber mutation in position 215.