Protein family review

This in an extract of a protein family review which first appeared in GenomeBiology, and is reproduced by permission of the publisher, BioMedCentral Ltd.

Authors:

Elisabeth Kruse, Norbert Uehlein and
Ralf Kaldenhoff

Institute of Botany, Department of Applied Plant Sciences, Darmstadt University of Technology, Schnittspahnstraße 10, D-64287 Darmstadt, Germany

Correspondence:

Ralf Kaldenhoff.

Email:

kaldenhoff@bio.tu-darmstadt.de

Read the full article

Subscribers to GenomeBiology may view the full version of this review article online at www.genomebiology.com

Published:

28 February 2006

The aquaporins

Summary

Water is the major component of all living cells, and efficient regulation of water homeostasis is essential for many biological processes. The mechanism by which water passes through biological membranes was a matter of debate until the discovery of the aquaporin water channels. Aquaporins are intrinsic membrane proteins characterized by six transmembrane helices that selectively allow water or other small uncharged molecules to pass along the osmotic gradient. In addition, recent observations show that some aquaporins also facilitate the transport of volatile substances, such as carbon dioxide (CO2) and ammonia (NH3), across membranes. Aquaporins usually form tetramers, with each monomer defining a single pore. Aquaporin-related proteins are found in all organisms, from archaea to mammals. In both uni- and multicellular organisms, numerous isoforms have been identified that are differentially expressed and modified by post- translational processes, thus allowing fine-tuned tissue-specific osmoregulation. In mammals, aquaporins are involved in multiple physiological processes, including kidney and salivary gland function. They are associated with several clinical disorders, such as kidney dysfunction, loss of vision and brain edema.

Frontiers

Since the description of the first aquaporin by Peter Agre and his colleagues, which was rewarded with the Nobel Prize for Chemistry in 2003, much information on the physiological significance of these channel proteins has accumulated. Additional functions in osmoregulation and metabolite transport have been attributed to this large and multifunctional protein family, and new physiological functions will probably be found in the future. As more biological roles of aquaporins are discovered, their potential in medicine, pharmacology and agrobiotechnology is also becoming clear.

Our knowledge of the structural determinants of the pore's selectivity will enable the development of channel-modulating agents for therapy. Detailed studies of aquaporin gene expression and regulation will lead to a more refined understanding of the involvement of aquaporins in pathophysiological processes.

Integration of data from studies in vitro and in intact plants will provide a more complete picture of the interaction and regulation of aquaporins in plants. Insight into the mechanisms of regulation with regard to subcellular distribution, heterotetramerization or other means of regulation will improve our understanding of water control and solute homeostasis in plants. This will help to develop plants with improved salt or drought resistance, more efficient water use and/or greater biomass production, through manipulation of the expression of individual aquaporins.

Three-dimensional structure of an aquaporin
subunit monomer (a ribbon model of NtAQP1,
a PIP1 protein from tobacco)

Topology of an aquaporin protein within the
membrane