AQUAPORIN SUPERFAMILY WITH UNUSUAL NPA BOXES: S-AQUAPORINS (SUPERFAMILY, SIP-LIKE AND SUBCELLULAR-AQUAPORINS)

Thirteen aquaporins have been identified in mammals. They all have highly conserved two asparagine-proline-alanine (NPA) boxes that are important for the formation of water permeating pore except AQP11 and 12, which have low homology (similar to 20%) with other AQPs and have poorly conserved NPA boxes. Such poorly conserved aquaporin-like sequences are widely found in database. Among them, SIPs from plants indeed have a water channel function and are localized in the cytosol, suggesting their roles in intracellular homeostasis. Based on their water channel function, at least in SIPs, and low homology with other AQPs, they will be subgrouped as a superfamily of AQPs. Accordingly, they are tentatively named S-aquaporins (superfamily, SIP-like and subcellular-aquaporins). Currently, their functional and biological grounds for an independent subfamily are not sufficient and may be reclassified into several subgroups in the future. The disruption of one of S-aquaporins, AQP11, produced neonatally fatal polycystic kidneys. AQP11 is also localized intracellularly. Further works on S-aquaporins will provide new insights into the functions and roles of aquaporins.