Protein Processing, Glycosylation, and Membrane Trafficking in the ER and Golgi

Key Concepts in Cell Biology: ER and Golgi Function

Overview

This study guide covers essential concepts in protein processing within the endoplasmic reticulum (ER) and Golgi apparatus, focusing on glycosylation, protein folding, quality control, and membrane trafficking. These processes are fundamental to cell biology and the proper functioning of eukaryotic cells.

Roles of the ER and Golgi Apparatus in Protein Processing

Protein Folding

• Chaperones in the ER assist in the proper folding of newly synthesized proteins.

• Protein Disulfide Isomerase (PDI) catalyzes the formation and rearrangement of disulfide bonds, ensuring correct protein structure.

Example:

PDI helps immunoglobulins form the correct disulfide bridges necessary for their function.

Quality Control

• Unfolded Protein Response (UPR): Sensor molecules in the ER detect misfolded proteins and can shut down translation to prevent accumulation.

• ER-Associated Degradation (ERAD): Misfolded or unassembled proteins are exported to the cytosol and degraded by proteasomes.

Example:

UPR activation in response to heat shock prevents further synthesis of misfolded proteins.

Glycosylation: Types and Mechanisms

General Kinds of Glycosylation

• N-linked glycosylation: Addition of an oligosaccharide to the nitrogen atom of asparagine residues. Starts in the ER and continues in the Golgi apparatus.

• O-linked glycosylation: Addition of an oligosaccharide to the oxygen atom on the hydroxyl group of serine, threonine, or rarely tyrosine residues. Occurs entirely in the Golgi apparatus.

Example:

N-linked glycosylation is critical for the stability and function of many secreted and membrane proteins.

Initial Glycosylation Occurs in the ER

• Early steps of N-glycosylation occur on the cytosolic surface of the ER membrane.

• Later steps take place in the ER lumen.

• Carbohydrate side chains initially have a core oligosaccharide structure: two units of N-acetylglucosamine, nine mannose units, and three glucose units.

Example:

The core oligosaccharide is essential for proper folding and quality control of glycoproteins.

Steps of Glycosylation

Mechanism of N-linked Glycosylation

• Glycosylation begins as dolichol phosphate, an oligosaccharide carrier, is inserted into the ER membrane.

• GlcNAc (N-acetylglucosamine) and mannose groups are added to the phosphate group.

• The growing core oligosaccharide is translocated to the ER lumen by a flippase.

• Once inside the lumen, more mannose and glucose are added.

• The completed core oligosaccharide is transferred from dolichol to the asparagine residue of the recipient protein.

• This process occurs co-translationally (as the protein is being synthesized).

• The core oligosaccharide attached to the protein is then trimmed and modified.

Example:

Defects in dolichol phosphate synthesis can lead to congenital disorders of glycosylation.

Summary Table: Types of Glycosylation

Type	Residue	Location	Core Structure
N-linked	Asparagine (N)	ER (initiation), Golgi (processing)	2 GlcNAc, 9 Mannose, 3 Glucose
O-linked	Serine, Threonine, Tyrosine	Golgi	Variable

Additional info:

• Glycosylation is essential for protein stability, trafficking, and cell-cell recognition.

• Defects in glycosylation pathways can result in various genetic diseases and cellular dysfunctions.