Which protein functions as a water channel and facilitates rapid movement of water across membranes?
Water permeability of plasma membranes exceeds lipid diffusion requiring dedicated proteinaceous pores. Aquaporin family comprising thirteen human isoforms forms homotetramers in kidney proximal tubules, red cells, and brain astrocytes each monomer about twenty eight kilodaltons six transmembrane helices arrangement N-terminus cytosolic with two half-helices HB and HE entering membrane containing invariant NPA motifs meeting at center forming aqueous pathway hourglass shape. Extracellular vestibule leads to aromatic arginine constriction formed by Arg195 His180 Phe56 limiting diameter to two p
Ref: Agre et al., Aquaporin Water Channels – Structure and Function, Nobel Lecture 2003.
