ABSTRACT
A novel category of major intrinsic proteins which share weak similarities with previously identified aquaporin subfamilies was recently identified in land plants, and named X (for unrecognized) intrinsic proteins (XIPs). Because XIPs are still ranked as uncharacterized proteins, their further molecular characterization is required. Herein, a systematic fine-scale analysis of XIP sequences found in flowering plant databases revealed that XIPs are found in at least five groups. The phylogenetic relationship of these five groups with the phylogenetic organization of angiosperms revealed an original pattern of evolution for the XIP subfamily through distinct angiosperm taxon-specific clades. Of all flowering plant having XIPs, the genus Populus encompasses the broadest panel and the highest polymorphism of XIP isoforms, with nine PtXIP sequences distributed within three XIP groups. Comprehensive PtXIP gene expression patterns showed that only two isoforms (PtXIP2;1 and PtXIP3;2) were transcribed in vegetative tissues. However, their patterns are contrasted, PtXIP2;1 was ubiquitously accumulated whereas PtXIP3;2 was predominantly detected in wood and to a lesser extent in roots. Furthermore, only PtXIP2;1 exhibited a differential expression in leaves and stems of drought-, salicylic acid-, or wounding-challenged plants. Unexpectedly, the PtXIPs displayed different abilities to alter water transport upon expression in Xenopus laevis oocytes. PtXIP2;1 and PtXIP3;3 transported water while other PtXIPs did not.
Subject(s)
Aquaporins/genetics , Evolution, Molecular , Magnoliopsida/genetics , Phylogeny , Polymorphism, Genetic/genetics , Populus/genetics , Amino Acid Sequence , Animals , Aquaporins/classification , Aquaporins/metabolism , Biological Transport , Droughts , Environment , Gene Expression Regulation, Plant/physiology , Magnoliopsida/metabolism , Magnoliopsida/physiology , Molecular Sequence Data , Multigene Family , Organ Specificity , Plant Leaves/genetics , Plant Leaves/metabolism , Plant Leaves/physiology , Plant Proteins/classification , Plant Proteins/genetics , Plant Proteins/metabolism , Plant Roots/genetics , Plant Roots/metabolism , Plant Roots/physiology , Plant Stems/genetics , Plant Stems/metabolism , Plant Stems/physiology , Populus/metabolism , Populus/physiology , Protein Isoforms , Sequence Alignment , Water/metabolism , Wood/genetics , Wood/metabolism , Wood/physiology , Xenopus laevis/genetics , Xenopus laevis/metabolismABSTRACT
A 13-kDa tobacco plasma membrane protein was isolated from two-dimensional electrophoresis gels. After microsequencing, RT-PCR techniques and cDNA library screening allowed for the cloning of two cDNAs. These cDNAs encoded for the subunit G of the vacuolar H+-ATPase, the first one identified in plants. Analysis of mRNA distribution showed a maximum level in the leaves and in the stem of the apical part of the tobacco plant. Heterologous functional complementation of the yeast mutant (deltavma10::URA3) was achieved with the two cDNAs. After fractionation of microsomal membranes on linear sucrose gradient, Western blots were performed using antibodies against recombinant protein and three peaks were identified: one which comigrated with the tonoplast marker and the others at slightly higher density corresponding to endoplasmic reticulum and to plasma membrane fractions.
