Study Notes on Cytoplasmic Membrane Systems

Chapter 8: Cytoplasmic Membrane Systems

Required Reading

  • Karp – Chapter 8

    • 8.1 – Overview of the Endomembrane System

    • 8.3 – The Endoplasmic Reticulum

    • 12.11 – Post-Translational Control

Additional Resources

  • Use the textbook's online resources:

    • Glossary, flashcards quizzes, practice questions (under assignments)

    • Review the following animations:

    • "Cell View Animation: Endomembrane System"

    • "Video: Animation of co-translational translocation"

8.1 Overview of the Endomembrane System

  • Protein Synthesis Location: All proteins begin synthesis on ribosomes in the cytosol, except for a few synthesized on the ribosomes of mitochondria and plastids in plants.

  • Fates of Synthesized Proteins:

    • End up in:

    • Cytoplasm

    • Organelle

    • Plasma membrane (or ER/Golgi)

    • Outside the cell (secreted)

Overview of Protein Sorting Pathways

  • Secretory Pathway (approx. 1/3 of all proteins)

    • Secreted proteins

    • Transmembrane proteins

    • Soluble proteins residing in the ER, Golgi complex, lysosomes, endosomes, vesicles, and vacuoles

  • Non-Secretory Pathways

    • Proteins synthesized by free ribosomes include:

    • Proteins destined to remain in the cytosol

    • Peripheral proteins of the cytosolic surface of membranes

    • Proteins transported to the nucleus

    • Proteins incorporated into peroxisomes, chloroplasts, and mitochondria

8.3 The Endoplasmic Reticulum (ER)

Overview of Protein Sorting Pathways

  1. Co-Translational Import (Cohesive Imports)

    • Utilized by ribosomes synthesizing polypeptides for export and intracellular compartments (e.g., lysosomes, endosomes, and Golgi).

    • Ribosomes attach early in translation to the rough ER membrane.

    • Translation completes on the rough ER. Once in the ER, proteins are further processed and sorted in the ER and Golgi.

    • Proteins lacking an ER Signal Sequence finish synthesis on free ribosomes in the cytoplasm.

    • Proteins with an organelle-specific Sorting Signal are imported into their respective organelle.

    • Cytoplasmic proteins lack Sorting Signals and remain in the cytoplasm.

  2. Post-Translational Import (Non-Secretory Pathway)

    • Involves proteins completed in the cytosol that are imported into organelles without initially associating with the ER.

Components of the Endomembrane System

  • Rough and Smooth Endoplasmic Reticulum:

    • Sites for protein (and lipid) synthesis, processing, and sorting.

  • Endosomes:

    • Carry and sort materials brought into the cell.

  • Lysosomes:

    • Digest ingested material and unneeded cellular components.

  • Vesicles (Transition, Transport, and Secretory):

    • Move molecules between compartments and the plasma membrane.

Exocytosis and Endocytosis

  • Exocytosis: Movement of materials outside the cell via secretory vesicles.

  • Endocytosis: Process by which cells absorb material (e.g., through vesicles).

    • Involves early endosomes, late endosomes, and lysosomes.

Overview of Exocytosis and Secretion

  • Secretory Pathways:

    • Constitutive secretory pathway: Constantly secretes materials.

    • Regulated secretory pathway: Stores materials until signals trigger their release.

Co-Translational Import Mechanism

  • Process Description:

    • Ribosomes synthesizing polypeptides for export become attached to ER membranes early in translation.

    • Polypeptide chains are transferred across the ER membrane as synthesis occurs.

    • An ER Signal Sequence directs the polypeptide/ribosome complex to the ER membrane.

Signal Recognition Particles and ER Signal Sequence

  • Signal Recognition Particle (SRP):

    • Composed of one 300 nucleotide RNA and 6 proteins.

    • Binds to the ER Signal Sequence, blocking further translation.

    • Guides the ribosome to the ER membrane where the translation continues and the signal sequence is recognized by the SRP receptor which facilitates translocation into the ER.

Key Steps in SRP Functioning:

  1. SRP binds to the signal sequence, halting further translation.

  2. SRP-ribosome complex binds the SRP receptor on the cytoplasmic surface of the ER membrane.

  3. The signal sequence is inserted into the translocon (channel protein in the membrane).

  4. The signal peptidase cleaves the ER Signal Sequence, initiating folding.

  5. The completed polypeptide is released into the ER lumen once synthesis is complete.

Protein Modifications in the ER

Protein Folding and Chaperones

  • Chaperones (e.g., BiP):

    • Prevent aggregation of polypeptides by binding to hydrophobic regions.

    • Guide folding and ensure the stability of protein structures.

  • Calnexin and Calreticulin:

    • Lectins involved in protein folding processes.

Post-Translational Modifications

  • N-linked Glycosylation:

    • Addition of oligosaccharides to asparagine residues during translation (co-translational glycosylation).

    • Modifications in the Golgi include processing O-linked sugars attached to serine and threonine.

  • Disulfide Bond Formation:

    • Facilitated by Protein Disulfide Isomerase (PDI) in the ER, crucial for protein structure stabilization.

Lipid Synthesis and Membrane Biogenesis

  • Lipid Functions:

    • Essential for membrane biogenesis and asymmetry.

    • Synthesized fatty acids are transferred into ER membranes by phospholipid translocators (flippases).

  • Distinct Lipid Compositions:

    • Derived from organelle-specific enzymes that can modify lipids, as well as selective budding vesicles and lipid transfer proteins.

Unfolded Protein Response (UPR)

  • Activated Under Stress:

    • Sensor molecules in the ER detect misfolded proteins and activate UPR.

    • Actions include:

    • Phosphorylation of translation factors to inhibit protein synthesis.

    • Upregulation of ER-based chaperones, transport mechanisms, and protein degradation machinery (ER-associated degradation - ERAD).

  • ERAD Mechanism:

    • Misfolded proteins are exported to the cytosol for degradation by proteasomes.

Conclusion

  • Understand the roles and distinctions between free ribosome-synthesized proteins and those synthesized on the rough ER.

  • Recognize the importance of signal sequences and their functional implications in protein sorting.

  • Acknowledge the role of glycosylation, chaperones, and the unfolded protein response in ensuring cellular protein homeostasis and functionality.

  • Familiarize with lipid synthesis processes and their contribution to membrane architecture across various organelles.