Aquaporin

Function

Aquaporins are channel producing proteins which regulate the flow of water across the cell membrane. The image on the left shows the protein, 6 molecules of glycerol and one of beta-octylglucoside.^[1]

• Aquaporin-0 functions as water channel in lens fibers.
  
• Aquaporin-1 see details in Aquaporin-1.
  
• Aquaporin-2 function is to reabsorb water from urine in the kidney.
  
• Aquaporin-3 function is to promote glycerol permeability across cell membrane.
  
• Aquaporin-4 regulates water balance in the central nervous system.
  
• Aquaporin-5 is implicated in the forming of saliva, tears and pulmonary secretions.
  
• Aquaporin-Z is a major water channel in bacteria.
  
• Aquaglycerolporin (GLpf) is a water channel which can transport glycerol, polyalcohols, urea and other small solutes.
  

Disease

Mutations in aquaporin-2 cause diabitis insipidus. Mutations in aquaporin-0 in mice cause congenital cataracts. Aquaporin-4 is the primary autoimmune target of neuromyelitis optica.

Structural highlights

Aquaporins are made of α-helix bundles. The water transporting channel contains 2 restriction sites conferring an hourglass model to the channel. Two NPA motifs from opposite surfaces form one restriction. Another restriction is formed by a cluster of aromatic/arginine side chains which serves to weaken the hydrogen bonding between water molecules.