Intracellular Compartments and Protein Transport Notes

Chapter 15: Intracellular Compartments and Protein Transport

Learning Objectives

  • Understand the different organelles in cells and their functions.

  • Grasp how proteins are sorted into various organelles through three types of transport.

  • Define signal sequences and their role in protein localization.

  • Describe the structure and function of the rough endoplasmic reticulum (ER).

  • Comprehend the mechanisms of vesicular transport including pathways and destination identification.

    • Exocytosis: Process of vesicles transporting substances out of the cell.

    • Endocytosis: Mechanism by which substances are brought into the cell.

Overview of Cellular Organelles

  • The following organelles are compartments enclosed by selectively permeable membranes, each having distinct functions:

    • Nucleus: Contains the main genome, involved in DNA and RNA synthesis.

    • Endoplasmic Reticulum (ER): Responsible for lipid synthesis and protein distribution.

    • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

    • Lysosomes: Involved in intracellular degradation of waste.

    • Endosomes: Sort endocytosed materials.

    • Mitochondria: Conducts ATP synthesis via oxidative phosphorylation.

    • Peroxisomes: Oxidative breakdown of toxic molecules.

    • Chloroplasts (in plant cells): Engage in photosynthesis.

Protein Sorting Mechanisms

  • Proteins are transported into organelles via three primary mechanisms:

    1. Transport through Nuclear Pores: Small proteins can pass freely while larger proteins require specific receptors.

    2. Transport Across Membranes of Mitochondria and Chloroplasts: Proteins unfold to cross membranes, utilizing signal sequences and import receptors.

    3. Transport by Vesicles: Involves budding from one membrane and fusing with another, transporting proteins between compartments.

Signal Sequences

  • Signal sequences are short peptide sequences that direct proteins to their appropriate cellular compartments.

  • If a protein lacks a signal sequence, it remains in the cytosol and does not reach its intended destination.

Protein Transport into the Nucleus

  • Nuclear proteins are imported from the cytosol via nuclear pores which allow selective import.

  • Import receptors in the cytosol bind proteins which then interact with the nuclear pore complex to gain access to the nucleus.

  • The release of the cargo protein occurs due to the binding of Ran-GTP in the nucleus, which displaces the cargo from its receptor. Ran-GDP is maintained in the cytoplasm.

Protein Transport into Mitochondria and Chloroplasts

  • Proteins must unfold to cross the outer and inner mitochondrial membranes:

    • A signal peptide is recognized by import receptors on the mitochondria.

    • Proteins translocate through import channels facilitated by protein translocators.

    • Final maturation of proteins occurs in the mitochondrial matrix where signal peptides are often cleaved off.

Protein Transport via the Endoplasmic Reticulum (ER)

  • The synthesis of proteins destined for secretion or for the plasma membrane occurs in the rough ER:

    • An ER signal sequence is recognized by the Signal Recognition Particle (SRP) that directs the ribosome to the ER membrane for protein synthesis.

Vesicular Transport

  • Vesicular transport is crucial for moving materials within the cell:

    • Exocytosis: Secretory pathway for releasing proteins.

    • Endocytosis: Uptake of molecules from the extracellular environment.

  • Vesicles bud from one membrane and fuse with another, directed by specific proteins and labels.

Vesicle Budding

  • The assembly of a protein coat (e.g., clathrin-coated vesicles) drives the budding process.

  • Key components include:

    • Clathrin: Coats the vesicle to assist in selection and transport.

    • Dynamin: A protein that helps in pinching off the vesicle from the membrane.

Vesicle Docking

  • Successful docking to target membranes is facilitated by:

    • Rab Proteins: These act as markers for the vesicles to reach appropriate membranes.

    • Tethering Proteins: Assist in the initial contact between the vesicle and target membrane.

    • SNARE Proteins: Facilitate the fusion of the vesicle with the target membrane.

Types of Exocytosis

  1. Regulated Exocytosis: Secretory proteins are released in response to specific signals.

  2. Constitutive Exocytosis: Continuous addition of membrane lipids and proteins to the plasma membrane.

Quality Control in the Endoplasmic Reticulum

  • Proteins that accumulate in the ER improperly trigger the unfolded protein response (UPR):

    • Recognized by transmembrane sensors in the ER membrane.

    • Misfolded proteins can lead to adaptations such as cell apoptosis if not resolved.

    • Example in C. elegans: High glucose leads to misfolding and activation of UPR, potentially linking to diabetes.

Endocytosis Mechanisms

  • Different modes of endocytosis include:

    1. Phagocytosis: Engulfment of large particles or pathogens.

    2. Receptor-mediated Endocytosis: Specific uptake driven by receptor-ligand interactions.

    3. Pinocytosis: Non-specific uptake of fluids and macromolecules.

Endosome Functionality

  • Endosomes sort macromolecules destined for lysosomal degradation or recycling.

  • Lysosomes are specialized for digestion, with enzymes that are active only at low pH.

  • They play a critical role in breaking down worn-out organelles and materials brought in by endocytosis.

Medical Implications

  • The unfolded protein response can contribute to diseases such as diabetes.

  • Phagocytic cells play an essential role in defense against infection.

  • Defects in receptor-mediated endocytosis of cholesterol can lead to significant health issues.