Plant major intrinsic proteins - functional implications of expression and localisation studies

Plant water channels are members of the very old Major Intrinsic Protein (MIP) family, with representatives in bacteria, fungi, animals, and plants. The genome of the model plant Arabidopsis thaliana contains 35 MIP genes. Based on protein sequence comparisons, the MIP family has been divided in four subfamilies: the PIP (Plasma membrane Intrinsic Protein), the TIP (Tonoplast Intrinsic Protein), the NIP (NOD26-like Intrinsic Protein), and the SIP (Small basic Intrinsic Protein) subfamilies. In order to harmonise the names of MIP members in all plant species, a nomenclature has been proposed, which is now broadly accepted. The increasing number of sequences from various plant species, including non-vascular plants, angiosperms, and conifers, provides information concerning the evolution of the MIP family in plants. In particular, the highly conserved PIP subfamily is divided into two groups, PIP1 and PIP2. The relatively large number of PIPs (13 in Arabidopsis) and the high degree of conservation within this subfamily point to important roles for these proteins in plants and to the existence of a high selective pressure on genes of both the PIP1 and the PIP2 groups in evolution. The recently solved structures of a mammal aquaporin, AQP1, and a bacterial glycerol facilitator, GlpF indicate that the overall fold of all members of the MIP family is likely to be conserved. However, a comparison of the sequences of these proteins and that of plant MIPs from different subfamilies indicates that members of the different MIP subfamilies probably exhibit different substrate specificities. Interestingly, PIP1 and PIP2 proteins purified from spinach show clear structural differences, implying that members of the two different groups in the PIP subfamily have different transport properties. MIP genes have been shown to be regulated at the transcriptional level by abiotic and biotic factors. Microarrays have been designed in order to investigate the expression of all Arabidopsis MIP genes. Data from a study of the organ specificity of MIP genes are presented. Results of reverse genetics studies suggest that the major substrate of PIPs is water. Immunolabelling studies of two PIP1 isoforms in spinach, together with similar studies in other species, indicate that PIP1 homologues are involved in phloem transport and guard cell movements. Future work should address the roles of the PIP1 proteins in these cells, and possible modes of post-translational regulation in planta.