01.26.11 lecture 6 - intracellular compartments and transport i
Post on 22-Dec-2015
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Intracellular transport and compartments
1.Protein sorting: How proteins get to their appropriate destinations within the cell
2.Vesicular transport: How vesicles shuttle proteins and membranes between cellular compartments
Organelles import proteins by three distinct mechanisms
1.Transport from the cytoplasm into the nucleus through nuclear pores
2.Transport from the cytoplasm to organelles by protein translocators in the membrane
3.Transport from ER to other organelles occurs via vesicles
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
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
NPCs actively transport proteins bound for the nucleus
1.Proteins bind to nuclear transport receptors
2.Complex is guided to the pore by filaments
3.Pore opens, receptor + protein are transported in (uses GTP)
4.Receptor is shuttled back into the cytoplasm
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
QuickTime™ and aH.264 decompressor
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Soluble proteins cross the ER membrane into the lumen
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
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
Step 2
•Additional clathrin molecules bind - increasing curvature
•Dynamin assembles a ring around each clathrin-coated pit
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
Step 4
•The free vesicle sheds its coat of adaptin and clathrin
•Vesicles are transported to their destination on microtubules
QuickTime™ and aH.264 decompressor
are needed to see this picture.
Animation of clathrin assembly and disassembly around an endocytic vesicle
SNAREs are proteins that target vesicles to specific compartments
t-SNARES are on target
compartments
v-SNAREs are on vesicles