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01.25.10 Lecture 6 - Intracellular compartments and transport I

01.25.10 Lecture 6 - Intracellular compartments and transport I. Intracellular transport and compartments. Protein sorting: How proteins get to their appropriate destinations within the cell Vesicular transport: How vesicles shuttle proteins and membranes between cellular compartments.

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01.25.10 Lecture 6 - Intracellular compartments and transport I

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  1. 01.25.10Lecture 6 - Intracellular compartments and transport I

  2. Intracellular transport and compartments • Protein sorting: How proteins get to their appropriate destinations within the cell • Vesicular transport: How vesicles shuttle proteins and membranes between cellular compartments

  3. Primary functions of the compartments within a cell

  4. Relative cellular volumes of the major membranous organelles

  5. The evolution of organelles: nuclear membranes and ER

  6. The evolution of organelles: mitochondria

  7. Organelles import proteins by three distinct mechanisms • Transport from the cytoplasm into the nucleus through nuclear pores • Transport from the cytoplasm to organelles by protein translocators in the membrane • Transport from ER to other organelles occurs via vesicles

  8. Signal sequences target proteins to their destinations

  9. Signal sequences are necessary and sufficient for protein targeting

  10. Mechanism 1: proteins enter the nucleus via nuclear pores • The nuclear envelope is a double membrane • Contiguous with the ER - both compartments share the same lumen • Perforated by nuclear pores

  11. The nuclear pore complex (NPC) is a selective molecular gate • Composed of ~100 different proteins • Small, water-soluble molecules pass freely, macromolecules must carry appropriate signal

  12. NPCs actively transport proteins bound for the nucleus • Proteins bind to nuclear transport receptors • Complex is guided to the pore by filaments • Pore opens, receptor + protein are transported in (uses GTP) • Receptor is shuttled back into the cytoplasm

  13. The nuclear envelope disassembles and reforms during each cell division

  14. Mechanism 2: protein translocation from cytoplasm to organelle • Proteins moving from the cytosol into the ER, mitochondria, chloroplasts, or peroxisomes • Protein movement is mediated by specialized proteins termed protein translocators • Unlike passage through nuclear pores, translocation requires unfolding or co-translational transport

  15. Proteins are unfolded during translocation into mitochondria

  16. The ER is a network of membrane sheets & tubules

  17. Active ribosomes may be in the cytosol or associated with the ER

  18. Ribosomes are directed to the ER by the SRP and ER signal

  19. Soluble proteins cross the ER membrane into the lumen

  20. Soluble proteins cross the ER membrane into the lumen

  21. Integration of transmembrane proteins into the ER membrane

  22. Multi-pass proteins use internal start-transfer sequences

  23. Mechanism 3: vesicular transport

  24. Transport vesicles • Continually bud off from and fuse to other membrane compartments producing a constant flux of material • Carry soluble proteins (in the lumen) and lipids & membrane proteins (in the bilayer) between compartments • Are transported along microtubules by motor proteins

  25. Vesicle budding is driven by assembly of a protein coat

  26. Clathrin-coated vesicles transport selected cargo molecules

  27. Complexes of clathrin form a basket around vesicles and help them to pinch from membranes

  28. Step 1 • Cargo molecules (red) bind to transmembrane cargo receptors • Cytoplasmic domains of receptors bind to adaptin (light green)which recruits clathrin • Clathrin clusters cargo/receptor/adaptin complexes and induces curvature to the membrane - clathrin-coated pit

  29. Step 2 • Additional clathrin molecules bind - increasing curvature • Dynamin assembles a ring around each clathrin-coated pit

  30. Step 3 • Dynamin rings constrict to “pinch” the membrane off • Dynamin is a GTPase and used the energy released from GTP hydrolysis to power this reaction

  31. Step 4 • The free vesicle sheds its coat of adaptin and clathrin • Vesicles are transported to their destination on microtubules

  32. Clathrin-coated vesicles transport selected cargo molecules

  33. Clathrin-coated vesicles transport selected cargo molecules

  34. Animation of clathrin assembly and disassembly around an endocytic vessicle

  35. SNAREs are proteins that target vesicles to specific compartments v-SNAREs are on vesicles t-SNARES are on target compartments

  36. SNARE proteins are important for membrane fusion • v-SNAREs and t-SNAREs bind tightly • Complexes bring the two membranes together to promote fusion

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