Aquaporins (AQPs) are a large family of integral membrane channel proteins
that facilitate the transport of water through the cell membrane. Aquaporins
are widely distributed and are involved in renal water absorption, generation
of pulmonary secretions, lacrimation and the secretion and reabsorption of
cerebrospinal fluid and aqueous humor. AQP0 is the most abundant endogenous
protein in the plasma membrane of lens fiber cells where it functions
not only as a water pore, but it is also involved in fiber-fiber adhesion and is
crucial for fiber cell structure and organization. AQP0 contains an additional
pore constriction, not seen in any other aquaporin structures, which may be
responsible for pore gating. The closed AQP0 pore holds just three water
molecules, which are spaced too far apart to form hydrogen bonds with each
other. The C-terminal domain of AQP0 undergoes extensive post-translational
modification, including many truncations, during lens aging due to the actions
of m-Calpain, proteases or non-enzymatic mechanisms. These truncation sites
may be involved in the development of cataracts.