Intracellular aquaporins: clues for intracellular water transport?

Aquaporins (AQPs) are usually present at the plasma membrane to regulate influx and outflow of water and small molecules. They are important for the regulation of water homeostasis for the cells and organisms. AQPs are also present inside the cell, at the membranes of intracellular organelles. The roles of such AQPs have not yet been established. They will be clues to clarify the mechanisms of water and small solutes movements inside the cell. Recently, a new AQP subfamily has been identified with highly deviated asparagine–proline–alanine boxes, signature sequences for AQP. With limited homology less than 20%, this subfamily will be a superfamily of AQPs. Accordingly, it was tentatively named “superaquaporin subfamily,” which is so far only present in multicellular organisms including plants, insects, nematodes, and vertebrates. Some superaquaporins are functionally water channels and localized intracellularly. AQP11, one of the two superaquaporins in mammals, has been shown to be important for the development of the proximal tubule as its disruption produced neonatally fatal polycystic kidneys in mice. Hence, recent identification of intracellular AQPs will open new areas of research on cell biology and expand the scope of AQPs.