The Arabidopsis tonoplast contains only a very small proportion of the N-glycoproteome, unlike the mammalian lysosomal membrane

N-glycosylation is one of the most important modifications of proteins synthesized by the secretory pathway. About one third of secretory proteins are N-glycosylated and their glycans are very often modified when proteins travel through the Golgi complex. It has been recently shown that the N-glycoproteomes of different eukaryotes mostly evolved after the phylogenetic divergences between plants, fungi, nematodes, insects, and vertebrates (Zelinska et al., 2012 Mol. Cell 46, 542-548). We have now analyzed in silico the percentage of N-glycosylated proteins in the tonoplast or plasma membrane (PM) of Arabidopsis and have verified our predictions experimentally. The results show that the tonoplast is virtually devoid of proteins with Golgi-modified glycans and contains only a small proportion, if any, of the N-glycoproteome, which is instead enriched in the PM. Lysosomes are considered the animal equivalents of plant vacuoles, because of shared abundance of hydrolytic enzymes and position along the secretory pathway. However, our results show that a much higher percentage of N-glycoproteins is present on the lysosomal membrane than in the tonoplast and that the very few tonoplast N-glycoproteins are much less extensively glycosylated. Thus, lysosomes and vacuoles had a divergent evolution in the composition of their membranes. This questions the hypothesis that the extensive N-glycosylation of lysosomal membrane proteins has evolved as a protection against the luminal hydrolases that are abundant in this compartment. Supported by the Italian Ministry of Education, Universities and Research (PRIN2010CSJX4F) and the EU Training Network VaTEP (MRTN-CT-2006-035833).