Endoplasmic Reticulum and Protein Synthesis

Endoplasmic Reticulum and Protein Synthesis

Overview

• Discussed topics include:
    - Apoptosis and caspases
    - Endomembrane system
    - Vascular transport
    - Green fluorescent protein as a tracing tool for organelles

Types of Endoplasmic Reticulum (ER)

• Smooth ER:
    - Lacks ribosomes
    - Main function: lipid synthesis
    - Additional functions:
      - Production of some steroid hormones
      - Detoxification (e.g., drug metabolism in liver cells)
      - Detoxification example:
        - Medications such as antibiotics can be degraded by the liver; the entire dose often must be consumed for effectiveness due to liver metabolism.
    - Sequestration/Storage of Calcium:
      - Calcium ions are crucial for cell signaling.
      - The cell maintains low intracellular calcium levels by actively expelling calcium ions using pumps.
      - Importance of calcium exclusion explained: Calcium binds poorly to water, risking insolubility of proteins in the aqueous cellular environment.
      - Calcium can be bound by calcium-binding proteins or transported out of the cell.

• Rough ER:
    - Contains ribosomes, giving a “rough” appearance.
    - Main functions:
      - Protein synthesis, glycosylation (addition of carbohydrates to proteins), protein folding, quality control, and synthesis of membrane phospholipids.
    - Relationship with the nuclear envelope: Rough ER is attached to nuclear envelope.

Protein Synthesis Process

Overview of Central Dogma

• Central Dogma:
    - DNA is transcribed into RNA.
      - Transcription occurs in the nucleus for eukaryotes and in the cytosol for prokaryotes.
      - RNA undergoes processing:
        - Addition of a 5' cap and 3' poly(A) tail.
        - Removal of introns (non-coding sequences) producing mature mRNA.
    - The matured mRNA exits the nucleus into the cytosol through the nuclear envelope.

Translation Process

1. Initiation:
      - The small ribosomal subunit binds to the mRNA; the first codon, AUG, is recognized as the start codon.
      - tRNA carrying methionine (the first amino acid) binds to the small ribosomal subunit at the corresponding anticodon UAC.
      - The large ribosomal subunit then joins, forming a complete ribosome with designated sites:
         - A (Aminoacyl site): entry for tRNA;
         - P (Peptidyl site): where peptide bonds form;
         - E (Exit site): where uncharged tRNA exits.

2. Elongation:
      - tRNAs carrying corresponding amino acids enter the A site and form peptide bonds with the growing polypeptide at the P site.
      - The uncharged tRNA exits from the E site.
      - Elongation continues, adding amino acids to the chain until a stop codon is reached.

3. Termination:
      - Stop codons signal the end of translation, leading to the release of the completed polypeptide chain.
      - Polypeptides can vary significantly in length (e.g., 3 to 3000 amino acids).
      - Protein structure characterized by N-terminus (amino group) and C-terminus (carboxyl group).

Role of Ribosomes

• Free ribosomes synthesize:
    - Cytosolic proteins, peripheral membrane proteins, and proteins targeted to organelles (nucleus, mitochondria, chloroplasts).

• Ribosomes bound to ER produce:
    - Secreted proteins and integral membrane proteins.

Signal Sequences and Targeting

1. Proteins targeted to the ER:
      - Proteins entering the ER possess a signal sequence at their N-terminus, typically rich in hydrophobic amino acids.
      - The signal sequence directs proteins into the ER lumen via co-translational import.
      - Signal Recognition Particle (SRP) aids in recognizing and halting translation until proper ER targeting is achieved.
      - Translocon assists the polypeptide as it moves into the lumen, where chaperones help fold the protein.

2. Mitochondrial protein targeting:
      - Proteins targeted to mitochondria use the TOM complex (Translocase of the Outer Membrane), similar to SRP, ensuring proper entry into the mitochondrial matrix.

Post-Translational Modifications

• Proteins in the rough ER lumen can:
    - Be retained in the ER, or
    - Be transported to Golgi apparatus for further modification and sorting.

• The Golgi apparatus has two major sides:
    - Cis face: entry side for proteins from the ER.
    - Trans face: exit side where modified proteins leave, often through exocytosis.

Conclusion

• Understanding protein synthesis and the functions of smooth and rough ER is essential for discerning cellular processes and organelle interactions.

• Refer to recommended readings for deeper comprehension and examples of protein synthesis and ER functions.