Water channel required to facilitate the transport of water across membranes. Involved in freeze tolerance, osmotolerance and cell flocculation in liquid cultures. Is non-functional in most laboratory strains (By similarity). During exponential phase in rich medium and repressed in minimum medium, hyper-osmolar medium or in sporulating conditions. 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. This is a truncated version of aquaporin-2. A natural 11 bp deletion in position 109 leads to a frameshift, which disrupts the gene coding for this protein and produces two ORFs C1Q_01794 and C1Q_01798. A contiguous sequence for aquaporin-2 can be found in strain Sigma 1278B (AC P0CD89).