Vesicle Formation, Golgi Maturation, and Endocytosis

Vesicle Formation and ER-Golgi Transport

• Coated Vesicles: Contain multiple receptors, pulling in various cargoes that can be either receptor-bound or soluble. The specificity of receptors means certain cargoes are targeted, but soluble components can also be non-specifically captured.

• COP II Coated Vesicles: Originate from the Endoplasmic Reticulum (ER). While seeking specific receptor-bound cargo, they also capture soluble secretory proteins. These vesicles move towards the Golgi apparatus.

• Golgi Structure: Characterized by cis and trans Golgi networks. The cis network is the entry point where vesicles fuse into the Golgi, and the trans network is the exit point for vesicles leaving the Golgi.

• Vesicular Tubular Clusters (VTCs):

• Before reaching the cis-Golgi, COP II vesicles (now uncoated) fuse together into irregularly shaped structures known as vesicular tubular clusters.

• These clusters are transitional elements that precede the Golgi.

• ER Resident Protein Retrieval: A 'cleanup process' to prevent loss of essential ER proteins.

• KDEL Sequence: ER resident proteins are identified by a specific C-terminal sequence, KDEL (Lys-Asp-Glu-Leu).

• KDEL Receptors: The ER membrane contains KDEL receptor proteins.

• Backward Transport: If ER resident proteins with the KDEL sequence are accidentally transported to the VTCs or Golgi, KDEL receptors will bind them.

• COP I Vesicles: These specific receptors facilitate the packaging of KDEL-bound proteins into COP I coated vesicles, which then bud off and transport the proteins backward to the ER, ensuring they return to their proper location.