The Endomembrane System and Protein Trafficking

Endomembrane System and Protein Trafficking in Eukaryotic Cells

I. General Principles of Protein Trafficking

• Initial Protein Synthesis:

  • All proteins begin their synthesis on ribosomes located in the cytosol or cytoplasm.

• Targeting of Proteins:

  • Proteins destined for the Endomembrane System:

    • These proteins possess a specific ER signal sequence.

    • As synthesis proceeds, the ribosome (with the attached protein) containing this signal sequence attaches to the Rough Endoplasmic Reticulum (ER).

    • These proteins enter the endomembrane system.

    • Classes of Endomembrane Proteins:

      • Secretory Proteins: Proteins that are eventually secreted or released by the cell, moving out of the plasma membrane into the extracellular environment.

      • Integral Membrane Proteins: Proteins embedded in the membranes of the plasma membrane, ER, Golgi complex, or lysosomes.

      • If an integral membrane protein is destined for the ER, Golgi, lysosome, or plasma membrane, it is synthesized on the Rough ER.

  • Proteins not destined for the Endomembrane System:

    • These proteins do not have an ER signal sequence.

    • Protein synthesis continues entirely on ribosomes in the cytoplasm.

    • Potential Destinations:

      • Cytoplasm: Proteins that function directly in the cytoplasm (e.g., cytoskeletal proteins, protein kinases). They are made and operate within the cytoplasm.

      • Other Organelles (not part of the endomembrane system): Nucleus, peroxisome, chloroplast, mitochondria.

        • These proteins are also synthesized in the cytoplasm but possess distinct targeting sequences specific to their respective organelles (e.g., mitochondrial targeting sequence, chloroplast targeting sequence, nuclear localization signals).

II. Components of the Endomembrane System

• The endomembrane system consists of:

  • Endoplasmic Reticulum (ER): Specifically the Rough ER for protein synthesis mentioned above.

  • Golgi Apparatus (Golgi Complex).

  • Plasma Membrane.

  • Lysosomes.

III. The Secretory Pathway (Exocytosis)

• This pathway describes how proteins destined for secretion or as integral membrane proteins of the plasma membrane are trafficked.

• Steps:

  1. Protein Synthesis and ER Insertion: A protein with an ER signal sequence is synthesized on the Rough ER and inserted into the ER.

  2. Lumen: The internal, empty space within these organelles (ER, Golgi) is referred to as the lumen. Proteins destined for secretion or the extracellular face of the plasma membrane are effectively within the lumen or integrated into its membrane.

  3. Processing in the ER: Proteins undergo initial processing and modification within the ER lumen.

  4. Vesicle Formation: The processed protein is then packaged into a small, membrane-bound organelle called a vesicle.

  5. ER to Golgi Transport: The vesicle transports the protein and fuses with the Golgi apparatus.

  6. Further Processing in the Golgi: Proteins undergo additional modification and sorting within the Golgi apparatus, which consists of flattened stacks (cisternae).

  7. Exit from Golgi: As processing completes, the protein exits from the trans Golgi network (TGN) or the trans face of the Golgi apparatus, again packaged into a vesicle.

  8. Secretion/Plasma Membrane Insertion: This final vesicle travels to and fuses with the plasma membrane, releasing the protein to the outside environment (secretion) or inserting it as an integral membrane protein.

• Integral Membrane Protein Orientation:

  • During the entire pathway, the cytoplasmic part of an integral membrane protein always remains in the cytoplasm.

  • The part of the protein that was in the lumen of the ER and Golgi becomes the extracellular part of the protein once it reaches the plasma membrane.

  • This implies that the lumen of the ER and Golgi is functionally equivalent to the extracellular space.

IV. Protein Sorting at the Golgi Complex

• The Golgi apparatus plays a critical role in protein sorting.

• From the trans Golgi network, vesicles containing proteins can be targeted to multiple destinations within the endomembrane system, not just the plasma membrane.

• Alternative Destinations:

  • Lysosomes: Vesicles can traffic proteins (e.g., lysosomal enzymes, lysosome integral membrane proteins) to the lysosome, which is a key component of the endomembrane system.

  • Return to ER: Proteins that are supposed to function permanently within the ER are also trafficked by vesicles. These vesicles fuse back with the ER, returning the proteins to their correct location.

V. Endocytosis and Related Pathways

• Endocytosis: The process by which cells take in materials from the outside environment. This also involves and enters the endomembrane system.

• Pathway of Internalized Material:

  1. Endocytic Vesicle Formation: Material from outside the cell is internalized, forming an endocytic vesicle.

  2. Early Endosome: The endocytic vesicle typically fuses with a compartment called the early endosome.

  3. Late Endosome: Material then moves to the late endosome.

  4. Lysosome: From the late endosome, many materials are ultimately transported to the lysosome for degradation.

• Receptor Recycling:

  • Receptors internalized during endocytosis (e.g., from the plasma membrane) can be recycled.

  • They travel from the early/late endosome back to the trans Golgi network.

  • From the trans Golgi, they are again packaged into vesicles and transported back to the plasma membrane for reuse.

• Phagocytosis:

  • A specialized form of endocytosis involving the uptake of large particles.

  • This process forms a large endocytic organelle called a phagosome, which ultimately fuses with the lysosome for digestion.

• Autophagy (Recycling Damaged Organelles):

  • Damaged or unwanted cellular structures/organelles are enveloped by membranes.

  • These membrane-bound structures are then sent to the lysosome for degradation and recycling of their components.