Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 3601.25.10Lecture 6 - Intracellular compartments and transport IIntracellular transport and compartments1.Protein sorting: How proteins get to their appropriate destinations within the cell2.Vesicular transport: How vesicles shuttle proteins and membranes between cellular compartmentsPrimary functions of the compartments within a cellRelative cellular volumes of the major membranous organellesThe evolution of organelles: nuclear membranes and ERThe evolution of organelles: mitochondriaOrganelles import proteins by three distinct mechanisms1.Transport from the cytoplasm into the nucleus through nuclear pores2.Transport from the cytoplasm to organelles by protein translocators in the membrane3.Transport from ER to other organelles occurs via vesiclesSignal sequences target proteins to their destinationsSignal sequences are necessary and sufficient for protein targetingMechanism 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 poresThe nuclear pore complex (NPC) is a selective molecular gate•Composed of ~100 different proteins•Small, water-soluble molecules pass freely, macromolecules must carry appropriate signalNPCs actively transport proteins bound for the nucleus1.Proteins bind to nuclear transport receptors2.Complex is guided to the pore by filaments3.Pore opens, receptor + protein are transported in (uses GTP)4.Receptor is shuttled back into the cytoplasmThe nuclear envelope disassembles and reforms during each cell divisionMechanism 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 transportProteins are unfolded during translocation into mitochondriaThe ER is a network of membrane sheets & tubulesActive ribosomes may be in the cytosol or associated with the ERRibosomes are directed to the ER by the SRP and ER signalSoluble proteins cross the ER membrane into the lumenQuickTime™ and aH.264 decompressorare needed to see this picture.Soluble proteins cross the ER membrane into the lumenIntegration of transmembrane proteins into the ER membraneMulti-pass proteins use internal start-transfer sequencesMechanism 3: vesicular transportTransport 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 proteinsVesicle budding is driven by assembly of a protein coatClathrin-coated vesicles transport selected cargo moleculesComplexes of clathrin form a basket around vesicles and help them to pinch from membranesStep 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 pitStep 2•Additional clathrin molecules bind - increasing curvature•Dynamin assembles a ring around each clathrin-coated pitStep 3•Dynamin rings constrict to “pinch” the membrane off•Dynamin is a GTPase and used the energy released from GTP hydrolysis to power this reactionStep 4•The free vesicle sheds its coat of adaptin and clathrin•Vesicles are transported to their destination on microtubulesClathrin-coated vesicles transport selected cargo moleculesClathrin-coated vesicles transport selected cargo moleculesQuickTime™ and aH.264 decompressorare needed to see this picture.Animation of clathrin assembly and disassembly around an endocytic vessicleSNAREs are proteins that target vesicles to specific compartmentst-SNARES are on target compartmentsv-SNAREs are on vesiclesSNARE proteins are important for membrane fusion•v-SNAREs and t-SNAREs bind tightly•Complexes bring the two membranes together to promote
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