Analysis of a putative PSI receptor domain in plant inositol transporters

来自植物和动物的质膜定位的H+-肌醇同向转运体具有12个跨膜螺旋的共同结构，并且在预测的螺旋IX和x之间有一个大的细胞外环。这种细胞外环在其他密切相关的细菌和真菌中的H+-肌醇同向转运体以及植物张力质体中的H+-肌醇同向转运体中是不存在的。基于对酵母和非洲爪蟾表达系统的广泛分析，我们可以证明去除这个环结构域增加了肌醇摄取的速度，并且基于硅结构和系统发育分析，我们可以证明这个结构域代表了一个新的PSI（=丛蛋白，信号蛋白，整合蛋白）结构域。PSI结构域进行蛋白/蛋白相互作用，迄今为止几乎只在动物单跨膜结构域（I型）受体的细胞外区域发现，但在运输蛋白和植物中都没有发现。总之，我们的数据表明植物和动物H+-肌醇同调体共享细胞外蛋白伴侣的受体功能或调节结合位点。本研究的目的是鉴定与拟南芥AtINT2和AtINT4蛋白环域相互作用的蛋白，并阐明其潜在的生理过程和调控途径。

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.