Aquaporin

Aquaporins, also called water channels, are channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a variety of different bacteria, fungi, animal and plant cells contain aquaporins. While water can pass through the phospholipid bilayer of the cell membrane through simple diffusion, this is relatively slow because water is a polar molecule; aquaporins provide facilitated transport, allowing water to flow more rapidly into and out of the cell, in effect increasing the water permeability of the cell membrane.

Aquaporins have six membrane-spanning α-helical domains with both carboxylic and amino terminals on the cytoplasmic side. Two hydrophobic loops contain conserved asparagine–proline–alanine ("NPA motif") which form a barrel surrounding a central pore-like region that contains additional protein density.

The 2003 Nobel Prize in Chemistry was awarded jointly to Peter Agre for the discovery of aquaporins and Roderick MacKinnon for his work on the structure and mechanism of potassium channels.

Genetic defects involving aquaporin genes have been associated with several human diseases including nephrogenic diabetes insipidus and neuromyelitis optica.