Review
Glycobiology in the cytosol: The bitter side of a sweet world

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Abstract

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of “glycobiology in the cytosol” which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

Introduction

The involvement of carbohydrate moieties of the glycoconjugates in biological phenomena is essential in innumerable life forms ranging from bacteria to humans. For instance, N-glycosylation plays pivotal roles in various contexts including folding or inter- and intracellular trafficking of the proteins [1], [2]. The biosynthetic pathways of N-glycosylation are well understood and genetically-modified animal models enable us to assess the importance of specific sugar modifications in even greater detail (plants, invertebrates [3], [4], [5], [6], mice [7], [8], [9]). The cytosol is known to be involved in numerous glyco-related events, such as glycolysis, glycogen metabolism, biosynthesis of sugar nucleotides and hyaluronic acids. However, for example, insufficient attention has been paid to the occurrence of N-glycosyl proteins in the cytosol. This is especially the case after the elucidation of topological issues on major glycosylation pathways [9], [10]. Accordingly, such studies have not been included in the main stream of glycobiological discourse and some considers them as experimental artifacts. The subcellular location of glycoproteins by purely biochemical fractionation is difficult to discuss. There have been, nevertheless, numerous reports on the occurrence of cytosolic glycoconjugates, as well as lectins and glycan-modifying enzymes, which now fit within a biological context. In this review we present an overview of our current knowledge on the occurrence of glycoproteins/lipids, with particular emphasis on N-glycosyl proteins. We also report on glycosidases/glycoamidases and lectins which are involved in metabolism/recognition of N-glycosyl proteins and/or free N-glycans in the cytosol.

Section snippets

‘Primordial’ glycoconjugates

Because most of well-characterized glycosylation events are known to occur in the lumen of vesicles such as the ER or Golgi, it may be seen as unusual to find glycoconjugates in the cytosol. However, it is widely accepted that some of the early biosynthetic glycoconjugate intermediates (‘primordial’ glycoconjugates) are indeed formed on the cytosolic face of vesicles. For example, the biosynthesis of glucosyl ceramide (Glc-Cer), the precursor for the vast majority of glycolipids, is known to

Glycoprotein degradation by ERAD

The appearance of glycoconjugates in the cytosol should not be restricted to bona fide cytosolic glycosylation. They can also occur through changes in the “topology” of lumenal glycoproteins, i.e. from the lumenal side to the cytosol. One of the best known examples is the retrotranslocation of misfolded glycoproteins via the ER-associated degradation (ERAD) pathway.

In eukaryotes, the ER is where proteins are synthesized and proceed through the secretory pathway. Proteins are known to be

Examples of well characterized N-glycosylated proteins found in the cytosol

The occurrence of N-glycosylated proteins are normally believed to reside in the lumen or outside of the cells. The presence of N-glycosylated proteins in the cytosol was discussed above. Such reports of the presence of N-glycosylated proteins in the cytosol may provide truly convincing evidence that this can happen under specific conditions.

PNGase

PNGase is an enzyme that catalyzes the cleavage of linkages between the innermost GlcNAc and an asparagine residue (Asn). This cleavage releases the free N-linked glycan from the protein and the glycosylated Asn residue is converted into aspartic acid through a deamidation reaction [136]. The cytoplasmic enzyme activity was first described in various mammalian-derived cultured cell lines [137] and later was found to occur ubiquitously in yeast through to humans [98], [137], [138], [139], [140].

Perspectives

The main focus of this review has been to outline our current understanding of the biological significance of cytosolic events related to glycosylation, deglycosylation and glycan recognition events by lectins. Despite the abundance of reports, especially over the last decade, research examining cytosolic events is pressed to generate much interest among the glycobiological science community. This is even more prevalent for the biochemical or cell biology communities at large. A provocative

Acknowledgements

We express our sincere thanks to Mr. David Wayne Chapmon (RIKEN) for the extensive English editing. The research in our laboratory is supported by Grants-in-Aids from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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