Molecular Cell Biology Quiz Review

Protein Targeting Signals

  • Nuclear Localization Signal (NLS)

    • Destination: Nucleoplasm (inside the nucleus)

  • Nuclear Export Signal (NES)

    • Destination: Cytoplasm (export from the nucleus)

  • Signal Sequence

    • Destination: Endoplasmic Reticulum (ER) lumen

  • Signal-Anchor Sequence

    • Destination: ER membrane

  • ER Exit Signal

    • Destination: Golgi apparatus

  • Mannose-6-Phosphate (M6P) Tag

    • Destination: Lysosomal lumen

  • Transmembrane Domain

    • Destination: Lysosomal membrane

  • Vesicle Sorting Signal

    • Destination: Secretory vesicle

  • Plasma Membrane Targeting Signal

    • Destination: Plasma membrane

  • Mitochondrial Targeting Sequence (MTS)

    • Destination: Mitochondria

  • Peroxisomal Targeting Signal (PTS1/PTS2)

    • Destination: Peroxisome

Protein Pathways to Specific Locations

  • Cytoplasm:

    • Signal: No signal peptide (default location)

  • Nucleoplasm:

    • Signal: Nuclear localization signal (NLS)

  • From Nucleoplasm to Cytoplasm:

    • Signal: Nuclear export signal (NES)

  • ER Lumen:

    • Signal: Signal sequence

  • ER Membrane:

    • Signals: Signal-anchor sequences or stop-transfer sequences

  • Golgi Membrane:

    • Signals: ER exit signals and Golgi retention signals

  • Lysosomal Lumen:

    • Signal: Mannose-6-phosphate (M6P) tag

  • Lysosomal Membrane:

    • Signals: Specific transmembrane domains and lysosomal targeting signals

  • Secretory Vesicle:

    • Signals: Secretory pathway signals including signal peptides and vesicle sorting signals

  • Plasma Membrane:

    • Signals: Transmembrane domains and plasma membrane targeting signals

Steps to Move Protein from Golgi to Lysosome

  1. M6P Tag Addition:

    • Proteins in the Golgi receive a mannose-6-phosphate (M6P) tag for lysosomal targeting.

  2. M6P Receptor Binding:

    • M6P receptors in the trans-Golgi network bind to tagged proteins.

  3. Vesicle Formation & Transport:

    • Proteins are packed into clathrin-coated vesicles and transported to the late endosome.

  4. Release & Recycling:

    • Acidic conditions facilitate protein dissociation and receptor recycling back to the Golgi.

  5. Lysosome Delivery:

    • The late endosome matures and fuses with the lysosome, delivering the protein.

Molecular Motors and Their Functions

  • Dynein:

    • Filament: Microtubules
    • Direction: Retrograde (toward the minus end, cell center)

  • Myosin:

    • Filament: Actin
    • Direction: Various (mainly toward the plus end)

  • Kinesin:

    • Filament: Microtubules
    • Direction: Anterograde (toward the plus end, periphery)
Key Functions:
  • Dynein: Moves cargo toward the center of the cell.
  • Kinesin: Moves cargo toward the outer edges of the cell.
  • Myosin: Interacts with actin filaments, facilitates intracellular transport, and plays a role in muscle contraction.