-endoplasmic reticulum

  • By the end of the lecture, students should be able to:

    • Recognize the cellular localization of the ER relative to the nucleus and Golgi.

    • Distinguish the different forms and structures of the ER (RER vs. SER).

    • Classify the structure and function of the Golgi apparatus in the secretory pathway.

    • Identify examples of signaling groups and signal sequences.

    • Understand the experimental methodologies (e.g., Pulse-Chase) used to determine movement through the ER.

Overview of Cellular Structure and Compartmentalization

  • Eukaryotic cells evolved complex internal membranes to create intracellular compartments, or organelles, each with distinct microenvironments (pH, ion concentrations, and enzymes).

  • This compartmentalization allows incompatible chemical reactions to occur simultaneously within the same cell.

Major Cellular Components

  • Endosome: Involved in sorting material internalized by endocytosis.

  • Cytosol: The site of many metabolic pathways and protein synthesis (free ribosomes).

  • Peroxisome: Oxidative organelles that break down long-chain fatty acids.

  • Lysosome: Contains digestive enzymes for intracellular degradation.

  • Golgi Apparatus: Modifies, sorts, and packages proteins for secretion or delivery to other organelles.

  • Mitochondrion: The site of ATP synthesis via oxidative phosphorylation.

  • Nucleus: Contains the genome and is the site of DNA and RNA synthesis.

  • Plasma Membrane: The lipid bilayer surrounding the cell.

Detailed Description of the Endoplasmic Reticulum (ER)

  • The ER is an extensive, continuous membrane system that can constitute more than 50 \% of the total membrane in an animal cell.

  • Lumen (Cisternal Space): The internal space enclosed by the ER membrane. It is physically distinct from the cytosol.

  • The ER membrane is continuous with the outer membrane of the nuclear envelope, creating a direct link between the nucleus and the site of protein/lipid synthesis.

Subdivisions: Rough vs. Smooth ER

  1. Rough Endoplasmic Reticulum (RER)

    • Appearance: Flattened sacs (cisternae) studded with ribosomes on the cytosolic side.

    • Functions:

      • Co-translational translocation: Ribosomes dock onto the ER membrane via the translocon complex when a signal sequence is recognized.

      • Synthesis of integral membrane proteins, secreted proteins, and lysosomal enzymes.

      • Addition of N-linked oligosaccharides (glycosylation).

  2. Smooth Endoplasmic Reticulum (SER)

    • Appearance: A tubular network lacking ribosomes.

    • Functions:

      • Lipid Synthesis: Synthesis of fatty acids, phospholipids, and steroids (e.g., cholesterol, testosterone, estrogen).

      • Drug Detoxification: Involves enzymes like Cytochrome P450, which add hydroxyl groups to drugs to make them more water-soluble for excretion.

      • Calcium Storage: Sequestration of Ca^{2+} ions from the cytosol.

Sarcoplasmic Reticulum (SR)

  • A specialized form of SER found in muscle cells (skeletal and cardiac).

  • High concentration of Ca^{2+} pumps (SERCA) that actively transport calcium into the SR lumen.

  • Release of Ca^{2+} into the cytosol triggers myofibril contraction through interaction with troponin.

Protein Transport and Signal Sequences

  • Signal Recognition: A Signal Recognition Particle (SRP) binds to the signal sequence on a nascent polypeptide, pausing translation until the ribosome docks at the ER membrane.

  • Translocation: The protein enters the lumen or integrates into the membrane through a proteinaceous pore called a translocon.

  • Signal Peptidase: Cleaves the signal sequence once the protein has entered the ER.

  • Transmembrane Proteins:

    • Single-pass proteins utilize one start-transfer and one stop-transfer sequence.

    • Multi-pass proteins utilize alternating hydrophobic sequences to weave through the membrane.

Post-Translational Modifications and Quality Control

  • Glycosylation: Pre-assembled 14-sugar oligosaccharides are transferred from a lipid carrier (dolichol) to the side chain of Asparagine (Asn) residues.

  • Protein Folding: Molecular chaperones, such as Binding Protein (BiP), assist in folding and preventing aggregation.

  • ER-Associated Degradation (ERAD): Misfolded proteins are exported (retro-translocated) back to the cytosol, ubiquitinated, and degraded by the 26S proteasome.

Movement of Proteins: The Secretory Pathway

  • Proteins move from the ER → Golgi → Secretory Vesicles → Cell Surface.

  • COPII-coated vesicles: Carry cargo anterograde (ER to Golgi).

  • COPI-coated vesicles: Carry cargo retrograde (Golgi back to ER for recycling).

  • SNARE proteins: Ensure docking of transport vesicles with target compartment membranes.

Experimental Techniques: Pulse-Chase

  • Pulse: Cells are briefly incubated with radioactively labeled amino acids (e.g., ^{3}H-Leucine).

  • Chase: The radioactive medium is replaced with an unlabeled (cold) medium.

  • Detection: At specific intervals (e.g., 3, 15, 30, 120 minutes), cells are fixed and visualized via autoradiography to track the location of the radioactive protein wave as it moves through the secretory pathway.