IGE Lect 7 cell compartments and protein sorting

Essential Biomedical Sciences: Cell Compartments and Protein Sorting

Learning Outcomes

Students should be able to:

  • Identify the main intracellular compartments, describing their functions.

  • Explain how ribosomes can become fixed, and how this helps to determine the intracellular fate of synthesized proteins.

  • Describe the secretory-endocytic pathway and its role in cellular import, export, and processing.

Key Organelles in Eukaryotic Cells

  • Mitochondria: Powerhouses of the cell, responsible for energy production through respiration.

  • Nucleus: Contains genetic material, coordinates cell activities.

  • Ribosomes: Synthesize proteins; can be free in the cytosol or attached to the rough endoplasmic reticulum (ER).

  • Endoplasmic Reticulum (ER): Rough ER is studded with ribosomes for protein synthesis; smooth ER is involved in lipid synthesis.

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

  • Lysosomes: Contain enzymes for digestion of cellular waste and macromolecules.

  • Peroxisomes: Involved in lipid metabolism and detoxification.

  • Cytoplasm: Fluid medium of the cell in which organelles are suspended.

Protein Processing and Localization

  • Proteins are initially synthesized in the cytosol.

  • Proteins with ER signal sequences are directed towards the ER for processing and eventual trafficking to other compartments.

    • Signal Recognition: The N-terminal hydrophobic signal sequence guides the ribosome to the ER membrane where translation continues.

    • the SRP can block sites on the ribosome (E site to stop translation ending)

Mechanism of Ribosome Binding to the Endoplasmic Reticulum

  • Ribosomes can be free or membrane-bound:

    • Free Ribosomes: Synthesize cytosolic proteins or proteins targeted to the nucleus, mitochondria, or peroxisomes.

    • Bound Ribosomes: Synthesize proteins destined for secretion, incorporation into membranes, or for the lysosome.

  • The Signal Recognition Particle (SRP) binds to the signal sequence and guides the ribosome to the ER membrane, facilitating translocation.

The Secretory-Endocytic Pathway

  • The secretory pathway involves the transport of proteins from the ER to the Golgi apparatus and finally to their functional destinations via vesicles.

  • Proteins that are translated in fixed ribosomes populate this pathway, which connects through vesicle traffic.

Glycosylation of Proteins in the Endoplasmic Reticulum (ER)

  • Glycosylation is the attachment of carbohydrates to proteins, critical for protein folding and stability.

  • N-linked glycosylation: Carbohydrates are added to the nitrogen atom of asparagine residues.

  • O-linked glycosylation: Carbohydrates attach to the hydroxyl group of serine or threonine residues.

Endocytosis and Pathways of Internalization

  • Endocytosis encompasses different processes for cellular uptake:

    • Phagocytosis: Engulfing solid particles.

    • Pinocytosis: Uptake of liquids and small molecules.

    • Receptor-mediated endocytosis: Specific uptake based on receptor-ligand interactions.

  • Endosomes form after internalization, serving as sorting compartments that lead to lysosome maturation.

Conclusion

Understanding cell compartments and protein sorting is vital for comprehending cellular function and the processes of import, export, and intracellular processing. Mastery of the secretory-endocytic pathway furthers the grasp of how cells communicate and maintain homeostasis.