Endoplasmic reticulum glycosylation

The Endoplasmic Reticulum Golgi Apparatus Are the Major Sites of Glycosylation... 

Protein glycosylation occurs in the endoplasmic reticulum (ER) and Golgi compartments of the cell and involves a complex series of enzymatic reactions catalysed by membrane-bound glycosyltransferases and glyco-sidases. The two main types of protein glycosylation have very different biosynthetic pathways. 

Because membranes components participate in nearly every cell activity their structures are also dynamic and far from the equilibrium states that are most readily understood in biophysical terms. Newly synthesized bilayer lipids are initially associated with endoplasmic reticulum (Ch.3) whereas phospholipids initially insert into the cytoplasmic leaflet while cholesterol and sphingolipids insert into the luminal endoplasmic reticulum (ER) leaflet. Glycosylation of ceramides occurs as they transit the Golgi compartments, forming cerebrosides and gangliosides in the luminal leaflet. Thus, unlike model systems, the leaflets of ER membranes are asymmetric by virtue of their mode of biosynthesis. 

Comparison of two analytical approaches, atomic force microscopy (AFM) and quartz crystal microbalance, for studying the binding of Con A to glycosylated carboxypeptidase, demonstrated that both could determine the quantitative parameters characterizing the interaction.65 Quantitative analyses of the interaction of Calreticulin (CRT), which is a soluble molecular chaperone of the endoplasmic reticulum, with various... 

A subsequent study in 2002 of 27 families with a condition known as multiminicore disease (MmD) also linked mutations in SEPNl to disease pathology. Multiple mutations were identified in exons 1, 5, 7, 8, 10, and 11, and the authors also mentioned that this region (RSMD) had been previously linked to MmD. Minicores are lesions by histochemistry of mitochondrial depletion within muscle tissue. The first biochemical study of selenoprotein N aimed to identify the protein localization by immunohistochemistry and found that the primary protein product of several identified mRNAs (splice variants) was a 70 kDa protein present in the endoplasmic reticulum. Two potential ER targeting domains were shown to be present and the peptide expressed from the first exon was shown to be required for localization into the ER. This study also revealed that selenoprotein N was an integral membrane protein that is N-glycosylated. Expression analysis showed pronounced levels in embryonic tissue with a reduction after development and differentiation. 

Protein glycosylation occurs mainly on serine and asparagine residues,but can also occur on hydroxylysine and hydroxyproline (Scheme 15). Glycosylation is very important in the endoplasmic reticulum and Golgi apparatus and can be involved in cell signaling. Many of the membrane-bound proteins and excreted proteins are glycosylated. Protein glycosylation is important in all forms of eukaryotes. ... 

Asparagine is a site for N-linked glycosylation of proteins, a posttranslational modification that should be associated with the endoplasmic reticulum. 

Synthesis of dolichol pyrophosphate, a required cofactor in N-hnked glycosylation of proteins in the endoplasmic reticulum... 

Many proteins on the surface of the plasma membrane, and the majority of secreted proteins, contain oligosaccharide residues that are post-translationally added to the endoplasmic reticulum and in the Golgi apparatus (see p.230). By contrast, cytoplasmic proteins are rarely glycosylated. Glycoproteins can contain more than 50% carbohydrate however, the proportion of protein is generally much greater. 

Dtirr G, Strajde J, Plemper R, Elbs S, Klee SK, Catty P, Wolf DH, Rudolph HK (1998) The medial-Golgi ion pump Pmrl supplies the yeast secretory pathway with Ca2-f- and Mn2+ required for glycosylation, sorting, and endoplasmic reticulum-associated protein degradation. Mol Biol Cell 9 1149-1162... 

Knop M, Hauser N, Wolf DH (1996a) N-Glycosylation affects endoplasmic reticulum degradation of a mutated derivative of carboxypeptidase yscY in yeast. Yeast 12 1229-1238... 

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