Besides water: Functions of plant membrane intrinsic proteins and aquaporins

Abstract

Our view on the mechanism of membrane water transport has been renewed by the molecular and functional characterization of aquaporins. Further analysis of additional conductivities of the so-called MIPs (membrane intrinsic proteins), in which aquaporins form a subclass, could easily improve many concepts of plant physiology. Functional assays of MIPs revealed conductivity to and specificity for water and/or small solutes. The general biochemical mechanism of transport and selectivity was understood after pore structure analysis of aquaporin proteincrystals (Borgnia et al. 1999a,b). Aquaporins exhibit a characteristic conserved arrangement with six transmembrane helices linked by three extraand two intracellular loops, Nand C-terminal domains protruding into the cytoplasm and a highly conserved amino acid motif, asparagineproline-alanine (NPA), occurring twice in the pore region. Due to a supposedly greater necessity for fine tuned water control, plant aquaporins are particularly abundant with a greater diversity than the paralogs in metazoans (Johanson et al. 2001). In Arabidopsis, for example, 35 MIP like isoforms were predicted from genome analysis. Some of them are, however, assumed to be pseudogenes (Quigley et al. 2001). The plant aquaporins are classified into four major subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), Nodulin-26-like intrinsic proteins (NLMs, NIPs), and small basic intrinsic proteins (SIPs) (Johanson and Gustavsson 2002). The NLMs were the first aquaporins identified in plants, and are located in the peribacteroid membrane of symbiotic root nodules. Here they are believed to control transport of metabolites between the host cytosol and bacteria (Fortin et al. 1987). However, these proteins are also found in non-legume plants, and exhibit glycerol transport activity when expressed heterologously in Xenopus oocytes (Dean et al. 1999; Weig and