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Endomembrane System Overview

Endomembrane System Overview

  • The endomembrane system is a system in nearly all eukaryotic cells composed of organelles and structures that work together in the processing and transport of proteins and lipids.

Key Components of the Endomembrane System

  • Key Players:

    • Rough Endoplasmic Reticulum (Rough ER)

    • Golgi Apparatus

    • Membrane-bound structures called vesicles

    • Microtubules

    • Motor proteins

Analogous Example: The Pancreas

  • Function of the Pancreas:

    • Produces digestive enzymes and insulin, a signaling molecule crucial for blood sugar regulation.

    • Critical Health Implication:

      • Damage to insulin-producing cells can lead to Type 1 diabetes, potentially fatal without treatment.

Insulin Production Process

  • Sequence of Events:

    1. Ribosome Synthesis: A ribosome synthesizes insulin, moving through the ER.

    2. Insulin Transport: A group of insulin molecules travels from the ER to the Golgi apparatus through tubular extensions.

    3. Golgi Processing: Insulin molecules are processed and packaged into vesicles within the Golgi apparatus.

    4. Vesicle Transport: Motor proteins transport vesicles containing insulin to the cell membrane.

    5. Secretion: The vesicle membrane fuses with the cell membrane, releasing insulin into the bloodstream.

Protein Synthesis and Entry to the Endomembrane System

  • Initial Step:

    • Messenger RNA (mRNA) for insulin is exported from the nucleus into the cytoplasm where it is translated by ribosomes into a primary sequence of the insulin protein.

    • Gatekeeper Role:

      • A signal sequence of 20 amino acids at the start of the insulin protein directs its entry into the endomembrane system.

    • ER Entry:

      • The signal sequence binds to an RNA-protein particle, which then binds to a receptor on the ER membrane, guiding the insulin protein into the ER through a channel.

    • Modification in the ER:

    • Proteins inside the ER:

      • Remove the signal sequence.

      • Assist folding into correct structures.

    • Other proteins may receive short carbohydrate chains (glycosylation).

Inside the Golgi Apparatus

  • Protein Shipment:

    • Proteins are transported from ER to the Golgi in vesicles.

    • New Findings:

      • Recent studies suggest proteins travel through temporary tubes that extend from the ER along microtubules, rather than vesicles budding off.

  • Processing Steps in Golgi:

    1. Final Modifications:

      • Insulin is cut and reassembled from longer inactive form to shorter active form.

      • Other proteins may receive lipid additives.

      • All proteins get molecular bar codes (carbohydrates) that determine their destination.

    2. Sorting Mechanism:

      • Bar codes bind to receptors that sort proteins for specific destinations (e.g., those for secretion are separated from those for internal use).

Vesicle Transport Mechanism

  • Vesicles Role:

    • Contain processed proteins and other materials crafted in the Golgi and transport them to various parts of the cell or for secretion.

    • Insulin vesicles remain stored until triggered by sugar levels post-meals.

  • Targeting Mechanism:

    • Vesicle membranes are decorated with protein zip codes for specific binding to target receptors, facilitating fusion with destination membranes.

  • Motor Proteins (Kinesin):

    • Vesicles are moved along microtubule tracks by motor proteins that change shape to transport cargo.

    • Phosphorylation:

      • Kinesin undergoes phosphorylation (addition of (PO_{4}^{3-})) to facilitate movement; this leads to conformational changes allowing kinesin to 'walk' along microtubules.

Summary of Endomembrane System Processes

  • The endomembrane system orchestrates critical functions within eukaryotic cells:

    1. mRNA export from the nucleus.

    2. Protein synthesis begins in ribosomes.

    3. Signal sequence directs entry into the ER.

    4. Proteins fold inside the ER.

    5. Processed proteins move to the Golgi through connection tunnels.

    6. Golgi processes and sorts products before packing them into vesicles.

    7. Kinesin transports vesicles to their final destinations, where they release their cargo.

Vocabulary

  • Endomembrane System: A collection of machines, cytoskeletal components, and organelles for producing, processing, and transporting proteins and lipids.

  • Signal Sequence: A segment of amino acids that enables a protein to enter the endomembrane system.

  • Motor Protein: A protein that facilitates cellular movement through shape changes induced by phosphorylation or dephosphorylation.

  • Phosphorylation: The addition of a phosphate group, typically from ATP, that can alter protein conformation.

  • Dephosphorylation: The process of removing a phosphate group from a protein, reversing the functions of phosphorylation.