While some transport proteins facilitate diffusing by functioning as carrier proteins that alternate between different conformational states, others do so by forming hydrophilic trans-membrane channels that allow specific solutes — mainly ions — to move across the membrane directly. Let’s consider three kinds of trans-membrane protein channels — ion channels, porins, and aquaporins.

Trans-membrane proteins that allow rapid passage of specific ions are called ion channels. Despite their apparently simple design – just a tiny pore lined with hydrophilic amino acid side chains – ion channels are remarkably selective. Most allow passage of only one kind of ion, so separate channels are needed for transporting different varieties. This selectivity is remarkable given the small differences in size and charge among these ions. The underlying mechanism is not yet well understood; a reasonable hypothesis is that selectivity may involve both ion-specific binding sites and a constricted centre that serves as a size filter. The rate of transport is equally remarkable — in some cases, a single channel can conduct almost a million ions per second.

Regulation of ion movement across membranes plays an important role in many types of cellular communication. For example, the transmission of electrical signals by nerve cells depends critically on rapid, controlled changes in the movement of Na+ (sodium) and K+ (potassium) ions through their respective channels. These changes are so rapid that they are measured in milliseconds. In addition to such short-term regulation, most ion channels are also subject to longer-term regulation, usually in response to external stimuli such as hormones.

Compared with ion channels, the pores found in the outer membranes of mitochondria, chloroplasts, and many bacteria are somewhat larger and much less specific. These pores are formed by multi-pass trans-membrane proteins called porins. Bacterial porins are among the trans-membrane proteins whose structures have been determined by X-ray crystallography. A key feature revealed by this technique is that the trans-membrane segments of porin molecules cross the membrane not as an helix but as a closed cylindrical sheet called a barrel. The barrel has a water-filled pore at its centre. Polar side chains line the inside of the pore whereas the outside of the barrel consists mainly of non-polar side chains that interact with the hydrophobic interior of the membrane. The pore allows passage of various hydrophilic solutes, with the size limit for the solute molecules determined by the pore size of the particular porin.

For cells in at least some tissues, however, a specific kind of water movement is mediated by a family of channel proteins called aquaporins. Aquaporins do not account for all water movement across membranes. Instead, they facilitate the rapid movement of water molecules into or out of cells in specific tissues that require this capability. For example, the proximal tubules of kidneys reabsorb water as part of urine formation, and cells in this tissue have a high density of AQPs in their plasma membrane. The same is true of erythrocytes, which must be able to expand or shrink rapidly in response to sudden changes in osmotic pressure as they move through the kidney or other arterial passages. In plants, AQPs are a prominent feature of the vacuolar membrane, reflecting the rapid movement of water that is required to develop turgor.

Aquaporins may well be responsible for rapid transport of water in other cell types as well, but these are some of the better-characterised examples at present. Interestingly, prokaryotes appear not to contain aquaporins, probably because of their small size, and hence their large surface area/volume ratio, makes facilitated transport of water unnecessary.

All aquaporins described to date are integral membrane proteins with six helical transmembrane segments. In the case of AQP-1, the aquaporin found in proximal kidney tubules, the functional unit is a tetramer of four identical monomers.

The four monomers appear to associate side-by-side in the membrane with their 24 trans-membrane segments oriented to form a central channel lined with hydrophilic side chains.

The writer is associate professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also Fellow Botanical Society of Bengal.