Chapter 14.6

14.6 Directing Membrane Proteins and Cytosolic Materials to the Lysosome for Degradation

Major Functions of Lysosomes

  • Degradation Roles:

    • Major function of lysosomes is to degrade:

    • Extracellular materials taken up by the cell.

    • Intracellular components under specific conditions.

  • Delivery of Materials:

    • Materials to be degraded must be delivered to the lumen of the lysosome, which contains various degradative enzymes.

    • Example: Endocytosed ligands such as LDL particles dissociate from their receptors in late endosomes and subsequently enter lysosomal lumen.

Vesicular Trafficking Mechanism

  • The general vesicular trafficking mechanism discussed previously can be used to deliver contents of endosomal organelles to lysosomes for degradation.

Pathways for Degradation

  1. Degradation of Membrane Proteins:

    • Delivers membrane proteins to lysosome interior for degradation, utilizing an unusual vesicle that buds into the endosome's lumen, producing a multivesicular endosome.

  2. Autophagy:

    • Delivers cytosolic materials to lysosomal lumen for degradation by forming an autophagosome that envelops cytosolic material or organelles like peroxisomes or mitochondria.

Multivesicular Endosomes and Protein Segregation

  • Lysosomal Membrane Proteins:

    • Resident lysosomal membrane proteins (e.g., V-class proton pumps, amino acid transporters) function and remain protected from degradation by soluble hydrolytic enzymes in the lumen.

    • Delivered to lysosomal membrane by transport vesicles from the trans-Golgi network or endosome.

  • Membrane Protein Degradation:

    • Endocytosed membrane proteins that are to be degraded are fully transferred to lysosome interior by specialized mechanisms.

    • Degradation of cell-surface receptors is a common mechanism to control cellular sensitivity to extracellular signals (refer to Chapter 15).

Early Experimental Evidence

  • Evidence from electron micrographs shows membrane vesicles and fragments within endosomes/lysosomes.

  • Experiments in yeast indicated that endocytosed receptor proteins targeting the vacuole (yeast equivalent of lysosome) are associated with membrane fragments/small vesicles, suggesting specialized vesicle incorporation.

  • Transport vesicles and internal vesicles differ in topological directionality:

    • Transport vesicles bud out from the donor organelle to the cytosol.

    • Endosome vesicles bud inward into the lumen.

  • Mature endosomes with internal vesicles are called multivesicular endosomes; fusion with lysosome delivers internal vesicles for degradation.

Delivery and Fusion Mechanism

  • Sorting proteins determine retention on lysosomal surface (e.g., pumps, transporters) versus incorporation into internal vesicles for degradation.

  • Figure 14-31 outlines the delivery process:

    1. Vesicle for degradation buds from Golgi, moving towards the late endosome and becoming uncoated.

    2. Plasma membrane proteins are endocytosed and merge with late endosome, embedding in its membrane.

    3. Membrane proteins bud into the endosome, forming a multivesicular body.

    4. Merging with lysosome allows degradation.

ESCRT Protein Role

  • Inward Budding of Vesicles:

    • Many proteins identified in yeast mutations blocking membrane protein delivery to vacuole facilitate inward budding and are involved in multivesicular endosome formation.

    • Cargo proteins entering multivesicular endosomes usually tagged with ubiquitin, which serves as a signal for targeting.

    • Monoubiquitin tagging of cargo proteins occurs at plasma membrane, trans-Golgi network, or endosomal membrane.

  • ESCRT Complexes:

    • Membrane Hrs protein recruits protein complexes to the endosomal membrane:

    • ESCRT (Endosomal Sorting Complex Required for Transport) proteins, including ubiquitin-binding protein Tsg101, drive vesicle budding and cargo loading.

    • Pinching off occurs through formation of filamentous spiral structure, driven by ATPase enzymes (Vps4 disassembles ESCRT proteins for reuse).

Retrovirus Budding

  • Retroviruses like HIV use a budding process similar to inward budding of vesicles.

  • HIV Gag Protein:

    • Gag protein binds to plasma membrane; polymerizes into a structure resembling a vesicle bud. Mutational studies have shown the importance of terminal segments for membrane association and proper pinching off.

  • Importance of Tsg101 binding to Gag indicates mechanistic similarities between HIV budding and endosomal vesicle budding.

Autophagy Pathway

  • Introduction to Autophagy:

    • Cells recycle macromolecules during stress conditions such as starvation.

    • Begins with formation of a cup-shaped structure that envelopes cytosolic material or organelles like mitochondria, culminating in autophagosome creation.

Autophagic Mechanism

  1. Autophagosome Formation:

    • Originates from membrane-bounded organelle; may derive from Golgi fragments as indicated by yeast study mutations affecting both Golgi and autophagy.

    • Targeted engulfment happens with organelle integrity loss, marked by ubiquitin signals.

  2. Autophagosome Growth:

    • Requires membrane delivery via fusion of transport vesicles; Atg8 plays a key structural role in autophagosome formation and growth.

  3. Fusion with Lysosome:

    • Fusion with lysosome requires outer membrane proteins facilitating the process; proteolytic cleavage of Atg8 allows effective fusion.

Key Concepts Summary

  • Endocytosed membrane proteins for lysosomal degradation form vesicles within endosomes and fuse with lysosomes.

  • Autophagy acts both in cellular recycling and quality control by degrading dysfunctional organelles and pathogens.

Future Analysis

  • To access further study tools, visit Achieve for resources such as videos, animations, and quizzes.

Summary of Key Terms

  • Adapter Protein (AP) Complexes

  • Autophagy

  • Clathrin

  • ESCRT Proteins

  • Exocytosis

  • Multivesicular Endosomes

  • Rab Proteins

  • Receptor-Mediated Endocytosis

  • Trans-Golgi Network

  • Transport Vesicles

Review Questions

  1. What methods allow following intercompartmental transport? What basics do they share?

  2. Discuss the roles of coat proteins in vesicle budding and recruitment mechanisms.

  3. Explain the implications of brefeldin A (BFA) treatment on Golgi proteins.

  4. Discuss the role of EAGE in inhibiting anterograde transport in COPI pathways.

  5. Describe specificity in vesicle fusion, including regulation by GTPase switch proteins.

  6. Mechanistic role of NSF in membrane trafficking.

  7. Procollagen synthesis implications for the Golgi cisternal maturation model.

  8. Examples of retrieval sequences - their effects on protein transport.

  9. Adapter protein complexes' binding roles with clathrin.

  10. Describe the defect in I-cell disease related to lysosomal enzyme targeting.

  11. Compare lysosomal sorting and regulated vesicle packaging.

  12. Analyze membrane protein sorting revealed by budding of enveloped viruses.

  13. Examine pH roles in receptor-ligand interactions in endocytosis pathways.

  14. Compare endosomal budding and HIV budding mechanisms, discussing inhibitor design.

  15. Distinguish the endocytosis and autophagic pathways; outline autophagosome formation and fusion.

  16. Discuss LDL receptor mutation implications in familial hypercholesterolemia for endocytosis.