The tonoplast – where sweetness is dispensable

The proteolytic machinery of certain cellular organelles is potentially harmful to resident proteins. In mammals, the large luminal domains of integral lysosomal proteins are heavily glycosylated to protect them from degradation. In this issue of Journal of Experimental Botany (pages 1769–1781) Pedrazzini and co-workers reveal that plants are very different. Glycosylation is one of the most common protein modifications in all eukaryotes. Resident proteins of different intracellular organelles including the endoplasmic reticulum (ER), Golgi apparatus and hydrolysing compartments are often glycosylated at multiple sites with oligosaccharide chains of varying length and composition. The attached carbohydrates modulate protein stability, activity and trafficking and mediate protein–protein interactions. N-glycosylation, the most prevalent form of protein glycosylation in eukaryotes, is defined by the covalent linkage of an oligosaccharide (N-glycan) to selected asparagine residues of newly synthesized secretory proteins as they are translocated into the ER. Once attached to the polypeptide chain, the N-glycans play crucial roles in protein folding and quality control processes (Helenius and Aebi, 2001) – for example, they regulate a specific cell death event in Arabidopsis (de Oliveira et al., 2016). Immediately after the transfer to the protein, N-glycans are subjected to step-wise processing reactions that alter the composition of the N-glycan and by doing so control the fate of glycoproteins. As a consequence, the exposure of a defined glyco-code can promote protein folding or alternatively serve as a signal for the degradation of terminally misfolded or orphan glycoproteins. In principle, these essential N-glycan-dependent processes look highly conserved between animals and plants (Hong et al., 2012; Huttner and Strasser, 2012). By contrast, in the Golgi apparatus where the N-glycans acquire their final composition (referred to as complex N-glycans) N-glycan processing steps differ substantially in eukaryotes. A large variety of structurally and functionally diverse complex oligosaccharides are generated in the Golgi in mammals. In different plant species, on the other hand, only a limited number of complex N-glycan structures have been reported so far (Wilson et al., 2001). An outstanding question is therefore whether the reduced structural diversity of Golgi-processed N-glycans in plants also leads to fundamental differences in the biological role of these protein-linked carbohydrates. While most of the machinery and pathways involved in biosynthesis and processing of N-glycan structures have been identified, the study of the biological role associated with distinct plant N-glycan structures on individual proteins has proved challenging so far (Strasser, 2014). More detailed and comprehensive (glyco)proteomic approaches of different organelles are needed for understanding the structure and function of glycoproteins in a specific cellular environment. Now Pedrazzini et al. (2016) reveal that the Arabidopsis tonoplast is almost completely lacking glycoproteins with complex N-glycans and even proteins with ER-derived N-glycans are highly underrepresented.