Cell Biology: Golgi Apparatus Structure

Golgi Apparatus Structure

• The Golgi apparatus is a cell organelle with a folded membrane made of a phospholipid bilayer.
• The Golgi apparatus has two distinct sides:
  • Cis Golgi: Convex side facing the rough endoplasmic reticulum (RER).
  • Trans Golgi: Concave side facing the cell membrane.
• Cis Golgi:
  • Proteins synthesized in the RER are transported via vesicles to the cis Golgi.
  • Vesicles containing COP-II proteins direct movement from RER to cis Golgi (anterograde).
• Trans Golgi:
  • Proteins undergo post-translational modifications within the lumen.
  • Modified proteins are enclosed in vesicles and transported either to the cell membrane or fused with lysosomes.
• Lumen:
  • The Golgi lumen is where post-translational modifications of proteins occur.
  • Modifications include folding proteins and adding molecules like lipids and carbohydrates.
  • This process allows sequential processing of newly synthesized proteins.
• Lysosomes:
  • Lysosomes are cellular "garbage collectors" containing enzymes for breaking down damaged cellular debris.

Golgi Apparatus Function

• The Golgi apparatus processes and sorts proteins received from the endoplasmic reticulum (RER).
• Proteins move from RER to the cis Golgi to the trans Golgi through a process called cisternal progression.
• Protein Modification:
  • The Golgi lumen contains enzymes that facilitate protein modification.
  • Protein modifications include:
    • N-linked glycosylation: Sugars are trimmed and added to make the protein active and specific.
    • O-linked glycosylation: A sugar residue is added to the hydroxy group of serine or threonine amino acids to make the protein active.
    • Phosphorylation: Mannose is phosphorylated to activate lysosomal enzymes.
• Protein Recycling:
  • Worn-out proteins, taken up by cells through phagocytosis/endocytosis, form an endosome.
  • Endosomes fuse with lysosomes, releasing proteins back to the Golgi for modification and recycling.
• Protein Sorting:
  • The Golgi apparatus packages proteins and tags them for specific locations, including:
    • Lysosomes
    • Membrane proteins
    • Secretion
  • Proteins like clathrin coat vesicles and direct the destination of proteins.

Review Questions

• Question 1:
  • Ribosomes facilitate the translation of mRNA into proteins.
  • The process of worn-out proteins being invaginated into the cell is called endocytosis.
  • The trans-Golgi is the concave side.
  • Protein synthesis occurs in the rough endoplasmic reticulum.
• Question 2:
  • COP-II proteins direct the movement of vesicles from the RER towards the cis Golgi.
• Question 3:
  • A sugar residue in O-linked glycosylation is added to the oxygen atom of serine and threonine amino acids.
• Question 4:
  • Mannose-6-phosphate marks/tags proteins destined for lysosomal degradation.
• Question 5:
  • The Golgi apparatus is especially prominent in goblet cells of bronchial epithelium, plasma cells, and Paneth cells of the intestinal epithelium.

Golgi Apparatus Structure

• The Golgi apparatus is a folded membrane-bound organelle found within cells.
• It is composed of a phospholipid bilayer, similar to a coiled membrane.
• The Golgi has two distinct sides: the cis Golgi and the trans Golgi.
• The cis Golgi is located near the rough endoplasmic reticulum (RER) and acts as the receiving end.
• The trans Golgi is located on the opposite side, facing the cell membrane or lysosomes, and functions as the sending end after modifications.
• The cis Golgi is also known as the convex side, while the trans Golgi is also known as the concave side.

Golgi Apparatus Function

• The primary function of the Golgi apparatus is modifying and packaging proteins.
• Proteins synthesized in the RER undergo N-linked glycosylation, where a sugar molecule attaches to the protein's asparagine residue.
• After glycosylation, proteins are packaged into vesicles that bud off from the RER and move to the cis Golgi.
• Vesicles containing modified proteins are marked with COP2 proteins, signaling their transport to the Golgi.
• Within the Golgi, proteins undergo further modifications and sorting before being packaged into new vesicles for transportation to different destinations.
  • Some vesicles transport modified proteins to the cell membrane, where they can be either secreted outside the cell or become part of the cell membrane.
  • Other vesicles transport modified proteins to lysosomes, cellular organelles responsible for breaking down waste and worn-out cell parts.

Protein Transport

• Proteins travel from the rough endoplasmic reticulum (RER) to the Golgi apparatus via vesicles.
• COP2 proteins coat the vesicles and direct them from the RER to the cis side of the Golgi.
• Some proteins are transported back to the RER from the trans side of the Golgi.
• COP1 proteins coat the vesicles for retrograde transport to the RER.

Golgi Modification

• The Golgi apparatus modifies proteins as they move through its compartments.
• Protein Modifications:
  • Trimming: Sugars are removed from N-linked glycosylation that occurred in the RER.
  • O-linked Glycosylation: Sugars are added to the hydroxyl (OH) group of amino acids like serine and threonine.
  • Phosphorylation: A phosphate group is added to mannose sugar residues on the protein.
    • This is particularly important for lysosomal enzymes.

Destination Determination

• The Golgi modifies proteins to signal their destination:
  • Lysosomal Enzymes: Phosphorylated mannose directs proteins to be packaged into lysosomes.
  • Cell Membrane Proteins: Proteins destined for the cell membrane can be either incorporated or secreted.

Exocytosis

• Proteins destined to be secreted are released from the cell via exocytosis.

Protein Recycling

• Worn-out membrane proteins can be endocytosed into the cell and transported to the Golgi for modification and recycling.
• Endosomes are vesicles formed by endocytosis, transporting proteins to the Golgi for recycling.
• Lysosomes break down engulfed material.
• Phosphorylation of mannose promotes lysosomal enzyme synthesis to activate them.
• Clathrin proteins coat vesicles transporting proteins to the cell membrane.