Cellular Protein Trafficking: The Endomembrane System and Vesicular Transport

Posttranslational Import

While most proteins enter the ER during synthesis, some are transported after synthesis is complete. In posttranslational import, proteins synthesized in the cytosol are kept in an unfolded state by Hsp70 family chaperones to facilitate their passage through membrane pores. A specialized complex of ER membrane proteins, including Sec62, 63, 71, and 72, works alongside the Sec61 pore to target the protein for translocation. Once the polypeptide emerges into the ER lumen, the chaperone BiP recruits to the site and uses ATP hydrolysis to "pull" the polypeptide into the lumen.

Exam Tip: Remember the distinction between the two import modes. Cotranslational import involves the SRP and occurs during synthesis, whereas posttranslational import involves Hsp70 and Sec62/63/71/72 complexes and occurs after synthesis is complete.

The Secretory Pathway and Exocytosis

Once proteins are processed and folded in the ER, they are transported via vesicles to the Golgi apparatus. The Golgi acts as a distribution center, where proteins undergo further glycosylation (the addition of carbohydrate chains) and are sorted into vesicles at the trans-Golgi network (TGN).

From the Golgi, proteins move toward the plasma membrane through exocytosis, the process of releasing contents outside the cell. This process can be categorized into three types:

1. Constitutive Secretion: An unregulated, continuous process where vesicles move to the plasma membrane and fuse immediately.
2. Regulated Secretion: Vesicles accumulate near the membrane and only fuse in response to a specific signal, such as a transient increase in intracellular $Ca^{2+}$ levels.
3. Polarized Secretion: Exocytosis is restricted to a specific side or region of the cell surface.

During exocytosis, the internal (lumenal) surface of the vesicle becomes the external surface of the plasma membrane. This ensures that molecules like glycoproteins and glycolipids are correctly oriented to face the extracellular environment.

Endocytosis and Cellular Homeostasis

To balance the loss of membrane during exocytosis and to acquire external materials, cells utilize endocytosis. This process involves the invagination of the plasma membrane to form vesicles that internalize substances.

Forms of Endocytosis

• Phagocytosis: Often called "cell eating," this is the ingestion of large solid particles.
• Pinocytosis: Often called "cell drinking," this is the uptake of extracellular fluids and small solutes.

Once internalized, endocytic vesicles typically fuse with early endosomes. As these endosomes mature into late endosomes, they eventually fuse with lysosomes to facilitate the digestion of the internalized material.

The steady-state composition of the plasma membrane is a dynamic equilibrium maintained by the balance between exocytosis and endocytosis. This balance is crucial for nutrient uptake, the recycling of membrane receptors, and the maintenance of overall cellular homeostasis.

Summary Table: Protein Trafficking Directions