The Golgi Apparatus

• The major processing activity is glycosylation: the adding of sugar molecules to form glycoproteins.
• Some of these will eventually end up as integral membrane proteins embedded in the plasma membrane.
• Other proteins moving through the Golgi will end up in lysosomes
• or be secreted by exocytosis (e.g., digestive enzymes).

  Link to discussion of the paths taken by proteins when they leave the Golgi.

• In some cells, e.g., mucus-secreting cells in epithelia, the amount of carbohydrate so far exceeds that of the protein that the product is called a mucopolysaccharide.
• In plant cells, the Golgi secretes the cell plate and cell wall.
• Small peptides, e.g., some hormones and neurotransmitters, are too small to be synthesized directly by ribosomes. Instead, the ribosomes on the ER synthesize a large precursor protein that is later cut up into small peptide fragments as it traverses the Golgi.

  Example: proopiomelanocortin - a polypeptide of 265 amino acids from which are cut

  • ACTH
  • alpha and beta MSH
  • beta-endorphin
  • and others.

The Golgi consists of a stack of membrane-bound cisternae located between the endoplasmic reticulum and the cell surface.

• to sort out the processed proteins and send them on to their destinations while
• reclaiming processing proteins (e.g., glycosylases) for reuse.

The Outbound Path

• Transition vesicles pinch off from the surface of the endoplasmic reticulum carrying
  • integral membrane proteins
  • soluble proteins awaiting processing
  • processing enzymes
• Pinching off requires that the vesicle be coated with COPII (Coat Protein II)
• The transition vesicles move toward the cis Golgi on microtubules.
• As they do so, their COPII coat is removed and they may fuse together forming larger vesicles.
• These fuse with the cis Golgi
• Sugars are added to proteins in small packets so many glycoproteins have to undergo a large number of sequential steps of glycosylation, each requiring its own enzymes.
• These steps take place as shuttle vesicles carry the proteins from cis to medial to the trans Golgi compartments.
• At the outer face of the trans Golgi, vesicles pinch off and carry their completed products to their various destinations.

The Inbound Path

The movement of cisternal contents through the stack means that essential processing enzymes are also moving away from their proper site of action.

Using a variety of signals, the Golgi separates the products from the processing enzymes that made them and returns the enzymes back to the endoplasmic reticulum.

This transport is also done by pinching off vesicles, but the inbound vesicles are coated with COPI (coat protein I)

How does a vesicle recognize its correct target?

• pairs of complementary SNARE proteins that enable each vesicle to recognize its correct target
• other proteins, NSF and SNAP, that mediate the fusion of the two membranes.

• NSF, a protein that mediates membrane fusion using the energy of ATP.
• SNAP = Soluble NSF Attachment Protein
• SNARE = Soluble NSF Attachment Protein Receptor
  • v-SNARE = vesicle SNARE
  • t-SNARE = target SNARE

v-SNARES and t-SNARES are both integral membrane proteins that bind specifically to each other by their surface domains.

• SNAP proteins join the complex and recruit
• several molecules of NSF that mediate membrane fusion using the energy of ATP

NSF and SNAP mediate fusion of all types of vesicles. The specificity of recognition of vesicle and its target is done by the SNAREs.

Another mechanism of Golgi traffic?

The Golgi is not a static cell organelle

• The Golgi disappears at the onset of mitosis (it breaks up into vesicles that fuse with the endoplasmic reticulum).
• By telophase of mitosis, the Golgi reappears - arising from the endoplasmic reticulum.