Question

Compared to the proteins in the trans Golgi compartment, those proteins in the cis Golgi compartment: are identical. contain fewer modifications. are never modified. all of these statements are correct.

Answer 1

contain fewer modifications

Proteins destined to be secreted from the eukaryotic cell are synthesized by ribosomes bound to the rough endoplasmic reticulum (RER). As the protein is synthesized, it is translocated across the RER membrane into the lumen of the RER where it folds into its final conformation. The ER then buds off vesicles that carry the protein to the golgi apparatus also called the golgi complex.

• The Golgi has a cis face (where vesicles enter) and a trans face (where vesicles leave). Thus the RER vesicles fuse with the cis compartment of the Golgi, releasing the protein into the Golgi lumen.
• The protein then moves through the Golgi complex to the trans compartment, being modified en route by the addition of carbohydrate residues.
• The synthesis of O-linked oligosaccharide occurs by the sequential addition of monosaccharide units to the newly synthesized protein as it passes through the golgi complex.

Glycosylation: First, N-acetylgalactosamine (GalNAc) is transferred to the relevant Ser or Thr residue of the protein by GalNAc transferase, an enzyme that uses UDP-GalNAc as the precursor. Other monosaccharides (galactose, N-acetylglucosamine (GlcNAc), sialic acid, fucose) are then added using the corresponding sugar nucleotides as precursors. The exact type and number (up to about 10) of monosaccharides added depends on the protein substrate.

• Finally, vesicles bud from the trans compartment and carry the glycosylated secretory proteins to the plasma membrane where the vesicles fuse, releasing their contents to the cell exterior.
• This fusion and extracellular release of protein is also called exocytosis.