Vegetative and Seed-Specific Forms of Tonoplast Intrinsic Protein in the Vacuolar Membrane of Arabidopsis thaliana

Abstract

Reports from a number of laboratories describe the presence of a family of proteins (the major intrinsic protein family) in a variety of organisms. These proteins are postulated to form channels that function in metabolite transport. In plants, this family is represented by the product of NOD26, a nodulation gene in soybean that encodes a protein of the peribacteroid membrane, and tonoplast intrinsic protein (TIP), an abundant protein in the tonoplast of protein storage vacuoles of bean seeds (KD Johnson, H Hofte, MJ Chrispeels [1990] Plant Cell 2: 525-532). Other homologs that are induced by water stress in pea and in Arabidopsis thaliana and that are expressed in the roots of tobacco have been reported, but the location of the proteins they encode is not known. We now report the presence and derived amino acid sequences of two different TIP proteins in A. thaliana. a-TIP is a seed-specific protein that has 68% amino acid sequence identity with bean seed TIP; 'y-TIP is expressed in the entire vegetative body of A. thaliana and has 58% amino acid identity with bean seed TIP. Both proteins are associated with the tonoplast. Comparsons of the derived amino acid sequences of the seven known plant proteins in the major intrinsic protein family show that genes with similar expression pattems (e.g. water stress-induced or seed specific) are more closely related to each other than the three A. thaliana homologs are related. We propose that the nonoverlapping gene expression patterns reported here, and the evolutionary relationships indicated by the phylogenetic tree, suggest a functional specialization of these proteins. diated by carriers, allowing fluxes of solutes against the electrochemical gradient and channels that permit passive transport, i.e. down the electrochemical gradient. Because the vacuoles of different cell types can show dramatic differences in the nature and magnitude of the ion and metabolite exchange with the cytoplasm (e.g. transport of potassium in guard cells or of malate in the mesophyll of CAM plants), the tonoplasts most likely have different channels and carriers with specialized functions. Recently, published reports point to a small, but growing, family of homologous intrinsic membrane proteins from quite different organisms (22,28). This family has been termed the MIP4 family after its best characterized member, MIP from bovine lens fiber junctional membranes. MIP has been shown to form tetrameric structures (1) with channel activity in artificial lipid membranes (7). One of the MIP homologs, GLPF, in the inner membrane of Escherichia coli plays a role in the facilitated transport of glycerol. By analogy, it seems reasonable to postulate that the other proteins in this superfamily also form channels that are involved in passive transmembrane transport of ions and/or metabolites. The first member of this family to be identified in plants was NOD26 (24), a plant-encoded protein in the peribacteroid membrane of soybean root nodules infected with Rhizobium bacteroids. Recently, three ...

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Isolation of putative embryo-specific genes using stress induction of carrot somatic embryos

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Characterization of Overexpressed cDNAs Isolated from a Hormone-Autonomous, Radiation-Induced Tumor Tissue Line of Arabidopsis thaliana

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