A plasma membrane protein carries an oligosaccharide containing mannose (Man), g

Question

A plasma membrane protein carries an oligosaccharide containing mannose (Man), galactose (Gal), sialic acid (SA), and N-acetylglucosamine (GIcNAc). These sugars are added to the protein as it proceeds through the secretory pathway. First, a core oligosaccharide containing Man and GlcNAc is added, followed by Gal, Man, SA, and GlcNAc in a particular order. Each addition is catalyzed by a different transferase acting at a different stage as the protein proceeds through the secretory pathway. You have isolated mutants defective for each of the transferases, purified the membrane protein from each of the mutants, and identified which sugars are present in each mutant protein The fol llowing Table summarizes the results. Sugars present in the purified protein Gal Cell lacking (A) Oligosaccharide Man SA GlcNAc protein transferase (B) Galactose transferase+ (C) SA transferase (D) GlcNAc transferase + less than in normal cells 1. central Golgi cisternae 2. cis Golgi network 3. ER 4. trans Golgi network

Explanation / Answer

Oligosaccharide addition to secretary proteins will start as proteins will move from Endoplasmic reticulum (ER) to Golgi body (cis-central-trans) and finally toward plasma membrane. Oligosaccharide addition will be least if any glycosylating protein (enzyme) will be defective in ER and glycosylation will be maximum if defective enzyme’s location is trans Golgi (as order given above). Therefore location of mentioned enzymes will be as follows:

1.   Central Golgi cisternae – galactose transferase

2.   Cis-Golgi cisternae- GlcNAc transferase

3.   ER- Oligosaccharide protein transferase

4.   Trans- Golgi cisternae- SA transferase

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