Posttranslational Protein Modification: Biosynthetic Control Mechanisms in the Glycosylation of the Major Myelin Glycoprotein by Schwann Cells

Abstract: The posttranslational processing of the asparagine‐linked oligosaccharide chain of the major myelin glycoprotein (P₀) by Schwann cells was evaluated in the permanently transected, adult rat sciatic nerve, where there is no myelin assembly, and in the crush injured nerve, where there is myelin assembly. Pronase digestion of acrylamide gel slices containing the in vitro labeled [³H]mannose and [³H]fucose P₀ after electrophoresis permitted analysis of the glycopeptides by lectin affinity and gel filtration chromatography. The concanavalin A‐Sepharose profile of the [³H]mannose P₀ glycopeptides from the transected nerve revealed the high‐mannose‐type oligosaccharide as the predominant species (72.9%), whereas the normally expressed P₀ glycoprotein that is assembled into the myelin membrane in the crushed nerve contains 82.9–91.9% of the [³H]mannose radioactivity as the complex‐type oligosaccharide chain. Electrophoretic analysis of immune precipitates verified the [³H]mannose as being incorporated into P₀ for both the transected and crushed nerve. The high‐mannose‐type glycopeptides of the transected nerve isolated from the concanavalin A‐Sepharose column were hydrolyzed by endo‐β‐N‐acetylglucosaminidase H, and the oligosaccharides were separated on Biogel P4. Man₈GlcNAc and Man₇GlcNAc were the predominant species with radioactivity ratios of 12.5/7.2/1.4/1.0 for the Man₈, Man₇, Man₆, and Man₅ oligosaccharides, respectively. Jack bean α‐d‐mannosidase gave the expected yields of free Man and ManGlcNAc from these high‐mannose‐type oligosaccharides. The data support the notion that at least two α‐1,2‐mannosidases are responsible for converting Man₉GlcNAc₂ to Man₅GlcNAc₂. The present experiments suggest distinct roles for each mannosidase and that the second mannosidase (I‐B) may be an important rate‐limiting step in the processing of this glycoprotein with the resulting accumulation of Man₈GlcNAc₂ and Man₇GlcNAc₂ intermediates. Pulse chase experiments, however, demonstrated further processing of this highmannose‐type oligosaccharide in the transected nerve. The [³H]mannose P₀ glycoprotein with M_r of 27,700 having the predominant high‐mannose‐type oligosaccharide shifted its M_r to 28,500 with subsequent chase. This band at 28,500 was shown to have the complex‐type oligosaccharide chain and to contain fucose attached to the core asparagine‐linked GlcNAc residue. The extent of oligosaccharide processing of this down‐regulated glycoprotein remains to be determined.