Characterization of three sugar transporters, STP7, STP8 and STP12

The controlled distribution of sugars between assimilate exporting source tissues (制造并输出同化物的组织) and sugar consuming sink tissues (耗糖“库”组织) is a key element for plant growth and development. Monosaccharide transporters (单糖转运蛋白) of the SUGAR TRANSPORT PROTEIN (STP) family contribute to the uptake of sugars into sink cells (“库”细胞). Here we report on the characterization of STP7, STP8 and STP12, three previously uncharacterized members of this family in Arabidopsis thaliana. Heterologous expression (异源表达) in yeast (Saccharomyces cerevisiae; 酿酒酵母) revealed that STP8 and STP12 catalyze (催化) the high-affinity (高亲和) proton-dependent (质子依赖性) uptake of glucose (葡萄糖) and also accept galactose (半乳糖) and mannose (甘露糖). STP12 additionally transports xylose (木糖). STP8 and STP12 are highly expressed in reproductive organs (生殖器官) where their protein products might contribute to sugar uptake into the pollen tube (花粉管) and the embryo sac (胚囊). stp8.1 and stp12.1 T-DNA insertion lines developed normally, which may point towards functional redundancy with other STPs. In contrast to all other STPs, STP7 did not transport hexoses (己糖), but was specific for the pentoses (戊糖类) L-arabinose (L-阿拉伯糖) and D-xylose (D-木糖). STP7-promoter-reporter gene plants showed an expression of STP7 especially in tissues with high cell-wall turnover, indicating that STP7 might contribute to the uptake and recycling of cell wall sugars. Uptake analyses with radioactive (放射性的) L-arabinose revealed that eleven other STPs are able to transport L-arabinose with high affinity. Hence functional redundancy might explain the missing mutant phenotype of two stp7 T-DNA insertion lines. Together, the data complete the characterization of the STP family and present the STPs as new L-arabinose transporters for potential biotechnological applications.

在植物制造并输出同化物的组织和耗糖“库”组织之间的糖分布调控是植物生长和发育的关键因素。糖转运蛋白STP家族的单糖转运蛋白是植物“库”细胞吸收糖的主要蛋白。本文报道了拟南芥中STP家族之前未曾有过研究的三个基因STP7，STP8和STP12的功能。在酿酒酵母中的异源表达显示STP8和STP12会催化高亲和质子依赖性的葡萄糖吸收，并且接受半乳糖和甘露糖。STP12另外还转运木糖。STP8和STP12基因在生殖器官高表达，其表达的蛋白可能作用于把糖吸收到花粉管和胚囊中去。stp8.1和stp12.1 T-DNA插入株系发育正常，这可能是由于这两个STP蛋白与其余的STP存在功能冗余。与所有其余的STP蛋白不同的是，STP7并不转运己糖，但是特异转运戊糖类的L-阿拉伯糖和D-木糖。STP7启动子报告基因植物显示STP7蛋白在细胞壁周转较频繁的组织中表达，说明STP7可能作用于细胞壁糖的吸收和再循环。放射性标记的L-阿拉伯糖吸收分析显示11个其余的STP蛋白能够高亲和的转运L-阿拉伯糖。因此，功能冗余可能解释了两个stp7 T-DNA插入株系缺失突变体的表型。综上，本文的结果完整了STP蛋白家族的功能鉴定，并且表明STP作为新的L-阿拉伯糖转运蛋白能够成为潜在的生物技术应用。

通讯：Ruth Stadler (http://www.mpp.nat.uni-erlangen.de/forschung/stadler.shtml)

研究方向：拟南芥细胞与细胞间的通讯；糖运输蛋白在植物光合作用同化产物分配中的作用。

doi: https://doi.org/10.1104/pp.17.01493