Since they are walled off from those environments by membranes (and sometimes by walls, also), those membranes need to be permeable, and permeable in a controlled way.

Chemical compounds are moved across membranes in a variety of ways:

• Transport proteins bind and move small molecules, some ions, and some large proteins

• Endo- and exocytosis, or budding, of the membrane into or out of the cell move big molecules

• Well-defined pores constructed of protein allow spontaneous movement of water and small carbohydrates

• Similarly defined channels provide selective transport of ions, both positive and negative

In general, pores and channels need the ability to select substrates for passage, and some kind of switching.

The 2003 Nobel Prize in Chemistry was awarded to Peter Agre for the discovery and structural analysis of water channels, and Roderick MacKinnon for his high-resolution crystal structure of a potassium channel.

Aquaporins are a family of membrane proteins providing for rapid movement of water molecules across membranes.

• More than 150 members have been identified, some of which also transport sugars, glycerin, and other small molecules.

• Erythrocytes (red blood cells), kidney tubules, and the vacuolar membranes of plants are particularly rich in aquaporins

• Water flux through aquaporins can be as high as 3 x 10⁹ molecules per second

• H₃O⁺ must be excluded so that the charge gradient across the membrane is not altered

• The on-off switch is the osmotic gradient

In just the last ten years, structures of a few aquaporins have become available, from either X-ray diffraction or cryoscopic electron microscopy.

Human AQP-1 (1h6i), Side View	Top View

• The channel is composed of six helices, tilted about 30^o with respect to a perpendicular line through the membrane

• The whole channel thus has a right hand twist, previously an unknown fold

• In vivo, AQP-1 forms a tetramer through the membrane; that is, four channels running parallel to each other

• The central pore is about 2 A across at the narrowest point, and has an overall hourglass shape

  CPK Model of AQP-1

Two narrowing points are particularly important:

The Size and Charge Filters of AQP-1	CPK Model of the Filter Region

• The narrowest point is where the side chain of Arg195 pinches together with those of Phe56 and His180
  • The Arg provides a strong positive charge as a barrier to H₃O⁺

• The rest of the narrowing is provided by two NPA (Asn, Pro, Ala) motifs

• Molecular dynamics simulations suggest that the NPA motifs perform another crucial function: disrupting the hydrogen bonding chain that otherwise would form among the waters passing through the pore
  • Disruption of the chain occurs by reorientation of water molecules to H-bond to the Asn residues

  • This is crucial because such H-bonded chains are efficient proton conductors; thus, the effect of Arg195 is amplified
    

  • If the Asn are mutated to leucines, water flow stops