Characterization of Plasma Membrane MIP Proteins in Maize

Abstract

A large number of major intrinsic protein (MIP) homologs have been identified in plant species (Tyerman et al., 1999; Kjellbom et al., 1999). They are classified in two main groups according to their sequence identity with MIPs localized in either the plasma membrane (PIP, plasma membrane intrinsic protein) or the vacuolar membrane (TIP, tonoplast intrinsic protein). In addition, a number of sequences together with a MIP protein from soybean (Glycine max L.), NOD26, found in the symbiosome membrane surrounding nitrogen-fixing bacteria, form a third cluster. In maize, more than 40 MIP cDNAs have been identified (Chaumont, Barrieu, Chrispeels and Jung, unpublished data) and sequence comparisons enabled us to assign a putative subcellular membrane location (TIP, PIP and NOD26-like protein). Evidence that MIPs form water channels (aquaporins) is mostly derived from experiments with heterologous expression systems such as Xenopus oocytes. Expression of aquaporins in this heterologous system increases considerably the osmotic water permeability (Pf) value of the plasma membrane. Most plant MIP homologues that have been tested for water transport activity have been found to be water channel or aquaporins. Some plant MIPs transport glycerol in addition to water. However, because the primary sequences of MIPs do not allow us to deduce which solute is being transported, functional testing is always necessary.