School of BioSciences, The University of Melbourne, Melbourne, VIC 3010, Australia.
Joint BioEnergy Institute, Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94702.
Proc Natl Acad Sci U S A. 2017 Apr 18;114(16):4261-4266. doi: 10.1073/pnas.1701894114. Epub 2017 Apr 3.
In plants, L-arabinose (Ara) is a key component of cell wall polymers, glycoproteins, as well as flavonoids, and signaling peptides. Whereas the majority of Ara found in plant glycans occurs as a furanose ring (Ara), the activated precursor has a pyranose ring configuration (UDP-Ara). The biosynthesis of UDP-Ara mainly occurs via the epimerization of UDP-xylose (UDP-Xyl) in the Golgi lumen. Given that the predominant Ara form found in plants is Ara, UDP-Ara must exit the Golgi to be interconverted into UDP-Ara by UDP-Ara mutases that are located outside on the cytosolic surface of the Golgi. Subsequently, UDP-Ara must be transported back into the lumen. This step is vital because glycosyltransferases, the enzymes mediating the glycosylation reactions, are located within the Golgi lumen, and UDP-Ara, synthesized within the Golgi, is not their preferred substrate. Thus, the transport of UDP-Ara into the Golgi is a prerequisite. Although this step is critical for cell wall biosynthesis and the glycosylation of proteins and signaling peptides, the identification of these transporters has remained elusive. In this study, we present data demonstrating the identification and characterization of a family of Golgi-localized UDP-Ara transporters in The application of a proteoliposome-based transport assay revealed that four members of the nucleotide sugar transporter (NST) family can efficiently transport UDP-Ara in vitro. Subsequent analysis of mutant lines affected in the function of these NSTs confirmed their role as UDP-Ara transporters in vivo.
在植物中，L-阿拉伯糖（Ara）是细胞壁聚合物、糖蛋白以及类黄酮和信号肽的重要组成部分。虽然植物糖中大多数 Ara 以呋喃糖环（Ara）的形式存在，但活性前体具有吡喃糖环构型（UDP-Ara）。UDP-Ara 的生物合成主要通过高尔基体内腔中 UDP-木糖（UDP-Xyl）的差向异构化发生。鉴于植物中发现的主要 Ara 形式是 Ara，UDP-Ara 必须离开高尔基体，才能被位于高尔基体胞质表面外侧的 UDP-Ara 变位酶转化为 UDP-Ara。随后，UDP-Ara 必须被运回到腔室中。这一步至关重要，因为介导糖基化反应的酶——糖基转移酶位于高尔基体腔室中，而在高尔基体中合成的 UDP-Ara 不是它们的首选底物。因此，UDP-Ara 向高尔基体的运输是一个先决条件。尽管这一步对于细胞壁生物合成以及蛋白质和信号肽的糖基化至关重要，但这些转运蛋白的鉴定仍然难以捉摸。在这项研究中，我们提供了数据，证明了在 中存在一组定位于高尔基体的 UDP-Ara 转运蛋白家族。应用基于蛋白脂体的转运测定法表明，核苷酸糖转运蛋白（NST）家族的四个成员可以在体外有效地转运 UDP-Ara。对这些 NST 功能受影响的突变株的后续分析证实了它们在体内作为 UDP-Ara 转运蛋白的作用。
