Combining various strategies to increase the coverage of the plant cell wall glycoproteome

Fine-tuning the specificity of boronate affinity monoliths toward glycoproteins through pH manipulation

Boronate affinity functionalized materials have recently drawn increasing attention due to their capability to selectively isolate and enrich glycoproteins and glycopeptides. As cheaper and more stable competitors to lectins, boronic acids are generally believed to yield a relatively wider spectrum specificity to glycoproteins. For better understanding and effective utilization of boronate affinity, it is necessary to establish if boronic acids exhibit lectin-like narrow specificity towards individual or a sub-class of glycoproteins. Here we report a pH manipulation strategy for fine-tuning the specificity of boronate affinity monoliths towards two sub-classes of glycoproteins, sialylated and nonsialylated glycoproteins. When the binding pH > the pK(a) of the boronic acid by one pH unit or more, the boronate affinity monolith preferentially binds to glycoproteins containing neutral sugars and excludes sialic acid containing glycoproteins due to electrostatic repulsion. When the binding pH < the pK(a) by one pH unit or more, the boronate affinity monolith binds to sialylated glycoproteins due to the exceptional binding affinity of the boronic acid towards sialic acid residues. The alternative specificity towards sialic acid and neutral sugar was first verified using an off-line combination of boronate affinity extraction with nano-ESI-Orbitrap MS/MS detection. The alternative specificity towards sialylated and nonsialylated glycoproteins was then demonstrated by means of off-line combination of boronate affinity extraction with MALDI-TOF MS. Finally, the developed approach was applied to the alternative extraction of intact sialylated and nonsialylated glycoproteins spiked in human serum.

Characterization of the arabinogalactan protein 31 (AGP31) of Arabidopsis thaliana: new advances on the Hyp-O-glycosylation of the Pro-rich domain

Proteins are important actors in plant cell walls because they contribute to their architecture and their dynamics. Among them, hydroxyproline (Hyp)-rich glycoproteins constitute a complex family of O-glycoproteins with various structures and functions. In this study, we characterized an atypical Hyp-rich glycoprotein, AGP31 (arabinogalactan protein 31), which displays a multidomain organization unique in Arabidopsis thaliana, consisting of a short arabinogalactan protein (AGP) motif, a His stretch, a Pro-rich domain, and a C-terminal PAC (PRP-AGP containing Cys) domain. The use of various mass spectrometry strategies was innovative and powerful: it permitted us to locate Hyp residues, to demonstrate the presence of carbohydrates, and to refine their distribution over the Pro-rich domain. Most Hyp were isolated within repeated motifs such as KAOV, KSOV, K(PO/OP)T, K(PO/OP)V, T(PO/OP)V, and Y(PO/OP)T. A few extensin-like motifs with contiguous Hyp (SOOA and SOOT) were also found. The Pro-rich domain was shown to carry Gal residues on isolated Hyp but also Ara residues. The existence of new type Hyp-O-Gal/Ara-rich motifs not recognized by the β-glucosyl Yariv reagent but interacting with the peanut agglutinin lectin was proposed. In addition, the N-terminal short AGP motif was assumed to be substituted by arabinogalactans. Altogether, AGP31 was found to be highly heterogeneous in cell walls because arabinogalactans could be absent, Hyp-O-Gal/Ara-rich motifs of different sizes were observed, and truncated forms missing the C-terminal PAC domain were found, suggesting degradation in muro and/or partial glycosylation prior to secretion.

The secreted plant N-glycoproteome and associated secretory pathways

N-Glycosylation is a common form of eukaryotic protein post-translational modification, and one that is particularly prevalent in plant cell wall proteins. Large scale and detailed characterization of N-glycoproteins therefore has considerable potential in better understanding the composition and functions of the cell wall proteome, as well as those proteins that reside in other compartments of the secretory pathway. While there have been numerous studies of mammalian and yeast N-glycoproteins, less is known about the population complexity, biosynthesis, structural variation, and trafficking of their plant counterparts. However, technical developments in the analysis of glycoproteins and the structures the glycans that they bear, as well as valuable comparative analyses with non-plant systems, are providing new insights into features that are common among eukaryotes and those that are specific to plants, some of which may reflect the unique nature of the plant cell wall. In this review we present an overview of the current knowledge of plant N-glycoprotein synthesis and trafficking, with particular reference to those that are cell wall localized.