The Endomembrane System Membrane organelles – a dynamic, integrated network
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The Endomembrane System Membrane organelles – a dynamic, integrated network Materials are shuttled – small transport vesicles Bud from a donor membrane compartment Move in a directed manner Pulled by motor proteins Tracks – microtubules of the cytoskeleton At their destination – fuse with membrane of acceptor compartment Repeated cycles of budding and fusion ein Synthesis and Transport within the
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Protein Synthesis and Transport within the Cell. The Endomembrane System Membrane organelles – a dynamic, integrated network Materials are shuttled – small transport vesicles Bud from a donor membrane compartment Move in a directed manner Pulled by motor proteins - PowerPoint PPT Presentation
Transcript of The Endomembrane System Membrane organelles – a dynamic, integrated network
The Endomembrane SystemMembrane organelles – a dynamic, integrated network
Materials are shuttled – small transport vesiclesBud from a donor membrane compartmentMove in a directed mannerPulled by motor proteinsTracks – microtubules of the cytoskeletonAt their destination – fuse with membrane of acceptor compartment
Repeated cycles of budding and fusion
Protein Synthesis and Transport within the Cell
The Endomembrane System
Protein Synthesis and Transport within the Cell
The Endomembrane SystemTypical biosynthetic pathway
Proteins synthesised in the endoplasmic reticulumModified through the Golgi complexTransported to destination
Also a secretory pathway
Protein Synthesis and Transport within the Cell
The Endomembrane SystemSecretory activities – two types
ConstituitiveTransported from site of synthesis and secreted continuously
RegulatedMaterials to be secreted are stored in large, densely packed, membrane-bound secretory granulesDischarged following specific stimulusExamples: endocrine cells, acinar cells, nerve cells
Protein Synthesis and Transport within the Cell
The Endomembrane SystemMaterials moved out by secretory pathwaysMaterials moved in by endocyte pathway
From outside the cells to endosomes / lysosomes
Secreted Proteins – targeted by sorting signalsIn the sequence orAttached oligosaccharide
Protein Synthesis and Transport within the Cell
The Endomembrane SystemAutoradiography
Example: Acinar cells – synthesis of digestive enzymes
Protein Synthesis and Transport within the Cell
The Endomembrane SystemAutoradiography
Cells overlayed with photographic filmExposed to radioisotopes in the sampleRadiolabeled
Protein Synthesis and Transport within the Cell
The Endomembrane SystemAutoradiography
Tissues – washed and transferred to unlabelled “pulse-chase”
Protein Synthesis and Transport within the Cell
The Endomembrane SystemBiochemical analysis of subcellular fractions
Technique to homogenize cellsCytoplasmic membranes fragmented and form vesiclesTypes separated using cell fractionationER and Golgi – form microsomes
Fractioned into rough and smooth
Protein Synthesis and Transport within the Cell
The Endomembrane SystemThe Endoplasmic Reticulum (ER)
Two categoriesRough (RER)Smooth (SER)
Composition of luminal or cisternal space uniqueRER – ribosomes bound to cytosolic surfaceSER – no ribosomesRER & SER – many complex differences
Protein Synthesis and Transport within the Cell
The Endomembrane SystemRER vs SER
RERExtensiveInterconnected sacs – cisternaeContinuous with nuclear envelope
SERTubularInterconnecting pipelines
Protein Synthesis and Transport within the Cell
The Endomembrane SystemSER functions
Skeletal muscleKidney tubulesSteriod producing cellsLiver
DetoxificationGlucose-6-phosphatase
Protein Synthesis and Transport within the Cell
The Endomembrane SystemRER functions – Protein SynthesisPolypeptides synthesized at two locales
1. On ribosomes attached to cytosolic surface of RER
Proteins secreted from the cellIntegral membrane proteinsSoluble proteins within ER, Golgi, lysosomes, endosomes
Protein Synthesis and Transport within the Cell
The Endomembrane SystemRER functions – Protein SynthesisPolypeptides synthesized at two locales
2. Other polypeptides – on free ribosomesReleased into the cytosol
EnzymesPeripheral proteins – plasma membranesProteins nucleusProtein mitochondria, peroxisomes
Protein Synthesis and Transport within the Cell
RER functions – Protein SynthesisHow are proteins synthesized at different sites?
Secretory proteins – signal sequencePolypeptide moves to cisternal space of ER
cotranslationally
Protein Synthesis and Transport within the Cell
RER functions – Protein SynthesisBegins after a messenger RNA binds to a free ribosome
Protein Synthesis and Transport within the Cell
RER functions – Protein SynthesisOther points to note
Interaction between signal sequence and SRP – a key stepSRP is the tag – enables specific binding of the complex to the ERBinding through at least two interactions
SRP – SRP receptorRibosome and translocon
Following bindingNascent peptide released from SRPInserted into translocon
On terminationRibsome releasedTranslocon channel reverts to narrow conformation
Protein Synthesis and Transport within the Cell
RER functions – Protein SynthesisProtein Synthesis and Transport within the Cell
RER functions – Processing of proteinsPolypeptide is moved from the translocon to the cisterna
Enzymes in the membrane Signal peptide removed – signal peptidaseCarbohydrates added – oligosaccharyltransferase
Protein Synthesis and Transport within the Cell
RER functions – Processing of proteinsIn the lumen
Molecular chaperones – BiP (binding protein)Recognize and bind unfolded proteinsAid in attaining native conformationMovement of the protein into the ER lumenProtein disulfide isomerase (PDI)
Protein Synthesis and Transport within the Cell
RER functions – Processing of proteinsMisfolded proteins
Transported to cytosolDestroyed in proteasomes
Quality control
Protein Synthesis and Transport within the Cell
