Abnormal Glycosphingolipid Mannosylation Triggers Salicylic Acid–Mediated Responses inArabidopsis   

Abstract

The Arabidopsis thaliana protein GOLGI-LOCALIZED NUCLEOTIDE SUGAR TRANSPORTER (GONST1) has been previously identified as a GDP-D-mannose transporter. It has been hypothesized that GONST1 provides precursors for the synthesis of cell wall polysaccharides, such as glucomannan. Here, we show that in vitro GONST1 can transport all four plant GDP-sugars. However, gonst1 mutants have no reduction in glucomannan quantity and show no detectable alterations in other cell wall polysaccharides. By contrast, we show that a class of glycosylated sphingolipids (glycosylinositol phosphoceramides [GIPCs]) contains Man and that this mannosylation is affected in gonst1. GONST1 therefore is a Golgi GDP-sugar transporter that specifically supplies GDP-Man to the Golgi lumen for GIPC synthesis. gonst1 plants have a dwarfed phenotype and a constitutive hypersensitive response with elevated salicylic acid levels. This suggests an unexpected role for GIPC sugar decorations in sphingolipid function and plant defense signaling. Additionally, we discuss these data in the context of substrate channeling within the Golgi.

KONJAC1 and 2 Are Key Factors for GDP-Mannose Generation and Affect l-Ascorbic Acid and Glucomannan Biosynthesis in Arabidopsis

“…First, kjc1-1 kjc2-1 was viable, which is distinct from the embryonic lethal phenotype of cyt1 (N-glycosylation) and csla7 (glucomannan) mutants (Lukowitz et al, 2001;Goubet et al, 2003). Second, there was no evidence of any constitutive cell death, as seen when mannosylation of GIPCs is defective (Mortimer et al, 2013). Third, there was no detectable defect in N-glycosylation, which resulted in perturbed cellulose synthesis in cyt1 (Lukowitz et al, 2001).…”

“…To examine the influence of kjc mutations on GIPC mannosylation, the sugar composition of GIPCs in calli of kjc mutants was determined by LC-MS/MS analysis. Although the proportion of GIPCs having two hexoses was slightly reduced in kjc1-1 and kjc2-1 mutants (Supplemental Figure 10), the change was minor in comparison with the gonst1 mutant, in which more than 70% of GIPCs lacked headgroup sugar (Mortimer et al, 2013).…”

“…This is supported by the observation that the cyt1 mutation, which is allelic to vtc1, perturbs the N-glycosylation of glycoproteins, resulting in an embryonic lethal phenotype (Lukowitz et al, 2001). Loss of GONST1, a Golgi-localized GDP-sugar transporter, specifically reduced the mannosyl residues of glycosyl inositol phosphorylceramides (GIPCs), causing severe dwarfism without affecting AsA and glucomannan biosynthesis (Mortimer et al, 2013). This suggests that the synthesis of GIPCs requires transport of GDP-Man into the Golgi apparatus by GONST1, but that of AsA and glucomannan do not.…”

“…In the gonst1 GDP-sugar transporter mutant, reduced mannosylation of GIPC occurs, which causes dwarfism and a constitutive hypersensitive response (Mortimer et al, 2013). To examine the influence of kjc mutations on GIPC mannosylation, the sugar composition of GIPCs in calli of kjc mutants was determined by LC-MS/MS analysis.…”

Endoplasmic reticulum: Where nucleotide sugar transport meets cytokinin control mechanisms

“…This could be partly due to targeting of individual NSTs to different compartments. However, other mechanisms, such as substrate channeling to a specific glycosyltransferase through direct interaction with an NST, could provide further functional specificity, 13,14 Apart from SLC35A3, transport of UDPGlcNAc in human cells is mediated by 2 other distantly related NSTs with low sequence identity, SLC35B4 and SLC35D2. 1 SLC35B4 is similar to Arabidopsis NSTs group IIIb, comprising characterized transporters of UDP-Glc and UDP-galactose (Gal) 15,16 and their uncharacterized homologs (Fig.…”