Concise Notes on Intracellular Compartments and Protein Transport

  • Membrane-enclosed Organelles

    • Key for cellular function and organization

  • Protein Sorting: Non-endomembrane Systems

    • Proteins transported into organelles via three mechanisms
    • Signal Sequences direct proteins to compartments
    • Nucleus Entry through nuclear pores
    • Proteins for Mitochondria/Chloroplasts must unfold to enter
    • Entry to Peroxisomes occurs from cytosol and ER, requires specific sequences

  • Protein Sorting: Endomembrane Systems

    • Proteins enter Endoplasmic Reticulum (ER) during synthesis
    • Soluble Proteins released into ER lumen; Membrane Proteins incorporated into RER
    • Transmembrane Protein Arrangement defined by start/stop signals

  • Vesicular Transport

    • Transport vesicles convey proteins/membranes between compartments
    • Budding driven by protein coat assembly (e.g., clathrin)
    • Docking relies on tethers and SNAREs for specificity

  • Secretory Pathways

    • Covalent Modifications (disulfide bonds, glycosylation) in ER
    • Protein quality controlled before exiting ER
    • Golgi Apparatus further modifies/sorts proteins
    • Exocytosis releases proteins from cell

  • Endocytic Pathways

    • Specialized phagocytic cells ingest larger particles
    • Pinocytosis: uptake of fluids/macromolecules
    • Receptor-mediated Endocytosis: selective uptake using receptors
    • Endosomes sort endocytosed materials; lysosomes digest intruded matter
    • Lysosomes are key sites for intracellular digestion

  • Key Concepts Review:

    • Organelles essential for cell function
    • Mechanisms of protein sorting into specific cellular compartments are diverse and nuanced.