Department of Molecular, Cellular, and Developmental Biology, University of Michigan, An Arbor, Michigan 48109-1048, USA.
Plant Cell. 2009 Dec;21(12):3792-802. doi: 10.1105/tpc.109.070284. Epub 2009 Dec 18.
Asn-linked glycans, or the glycan code, carry crucial information for protein folding, transport, sorting, and degradation. The biochemical pathway for generating such a code is highly conserved in eukaryotic organisms and consists of ordered assembly of a lipid-linked tetradeccasaccharide. Most of our current knowledge on glycan biosynthesis was obtained from studies of yeast asparagine-linked glycosylation (alg) mutants. By contrast, little is known about biosynthesis and biological functions of N-glycans in plants. Here, we show that loss-of-function mutations in the Arabidopsis thaliana homolog of the yeast ALG12 result in transfer of incompletely assembled glycans to polypeptides. This metabolic defect significantly compromises the endoplasmic reticulum-associated degradation of bri1-9 and bri1-5, two defective transmembrane receptors for brassinosteroids. Consequently, overaccumulated bri1-9 or bri1-5 proteins saturate the quality control systems that retain the two mutated receptors in the endoplasmic reticulum and can thus leak out of the folding compartment, resulting in phenotypic suppression of the two bri1 mutants. Our results strongly suggest that the complete assembly of the lipid-linked glycans is essential for successful quality control of defective glycoproteins in Arabidopsis.
天冬酰胺连接的聚糖，或糖码，为蛋白质折叠、运输、分拣和降解携带关键信息。在真核生物中，生成这种糖码的生化途径高度保守，由脂质连接的十四糖的有序组装组成。我们目前关于聚糖生物合成的大部分知识都是通过对酵母天冬酰胺连接糖基化（alg）突变体的研究获得的。相比之下，人们对植物中 N-聚糖的生物合成和生物学功能知之甚少。在这里，我们表明，酵母 ALG12 的拟南芥同源物的功能丧失突变导致不完全组装的聚糖转移到多肽上。这种代谢缺陷严重影响了 BRASSINOSTEROID 两种缺陷跨膜受体 bri1-9 和 bri1-5 的内质网相关降解。因此，过量积累的 bri1-9 或 bri1-5 蛋白会使内质网中保留这两种突变受体的质量控制系统饱和，从而可以从折叠隔室漏出，导致这两种 bri1 突变体的表型抑制。我们的结果强烈表明，脂质连接聚糖的完全组装对于拟南芥中缺陷糖蛋白的成功质量控制至关重要。
