Analysis of a putative PSI receptor domain in plant inositol transporters

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Plasma membrane-localized H+-inositol symporters from plants and animals share a common structure of 12 transmembrane helices and a large extracellular loop between the predicted helices IX and X. This extracellular loop is absent from the otherwise closely related H+-inositol symporters from bacteria and fungi and from H+- inositol symporters of the plant tonoplast. Based on extensive analyses in the yeast and Xenopus expression systems we could show that removal of this loop domain increased the rate of inositol uptake, and on the basis of in silico structural and phylogenetic analyses we could demonstrate that this domain represents a novel PSI (= plexin, semaphorin, integrin) domain. PSI domains perform protein/protein interactions and were so far found almost exclusively in the extracellular regions of single-transmembrane-domain (type I) receptors from animals, but neither in transport proteins nor in plants. In summary, our data suggest that plant and animal H+-inositol symporters share receptor functions or regulatory binding sites for extracellular protein partners. The aim of the proposed work is the identification of proteins interacting with the loop domains of the Arabidopsis AtINT2 and AtINT4 proteins and the elucidation of the underlying physiological process and regulatory pathways.