'Glyco-deglyco' processes during the synthesis of N-glycoproteins

For the past 15 years, it has appeared increasingly evident that the N-glycosylation process was accompanied by the release of oligomannoside type oligosaccharides. This material is constituted of oligosaccharide-phosphates and of neutral oligosaccharides possessing one GlcNAc (OS-Gn1) or two GlcNAc (OS-Gn2) at the reducing end. It has been demonstrated that oligosaccharide-phosphates originated from the cleavage by a specific pyrophosphatase, of non-glycosylated cytosolic faced oligosaccharide-PP-Dol and chiefly the Man5GlcNAc2-PP-Dol. The Man5GlcNAc2-P, as the main product, is recovered in the cytosolic compartment and is further degraded to Man5GlcNAc1 by as for yet not depicted enzymes. In contrast, OS-Gn2 produced from hydrolysis of oligosaccharide-PP-Dol (presumably as a transfer reaction onto water) when the amount of protein acceptor is limiting, are generated into the lumen of rough endoplasmic reticulum (ER). They are further submitted to processing alpha-glucosidases and rough ER mannosidase and are (mainly as Man8GlcNAc2) exported into the cytosolic compartment. This material is further degraded into a single component, the Man5GlcNAc1: Man alpha 1-2Man alpha 1-2Man alpha 1-3 (Man alpha 1-6)Man beta 1-4GlcNAc by the sequential action of a cytosolic neutral chitobiase followed by cytosolic mannosidase. Furthermore, OS-Gn1 could have a dual origin: on the one hand, they originate from OS-Gn2 by the cytosolic degradation pathway indicated above; on the other hand, we will discuss a possible origin from the degradation or remodeling of newly synthesized glycoproteins. Considered first as a minor phenomenon, these observations have lead to the concept of intracellular oligomannoside trafficking, a process which results from more fundamental phenomena such as the control of the dolichol cycle, and the so-called quality-control of glycoprotein. In this review, we would like to describe the evolution of ideas on the origin, intracellular trafficking and putative roles of these oligomannosides released during the N-glycosylation process. We propose that these early stage 'glyco-deglyco' processes represent a way of control of N-glycosylation and of the fate of N-glycoproteins.