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Lecture 5

Lecture 5 - Proteins in ER.docx

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Western University
Biology 2382B
Robert Cumming

Lecture 5 – Where do Proteins go after ER? Long path to get from protein just entering the ER until eventually its secreted from the cell Entire process is vesicle dependent Lots of vesicles moving backwards using retrograde mechanism, but the vesicle movement forward only happens from the ER lumen into the golgi network initially, not after Physical movement of the structures up – it changes the function of the proteins inside  The above is not a focous of the course though, focus on retrograde movement (returning things back to ER) and formation Golgi Complex  Consists of flattened, disklike, cisternae with no ribosomes o Has a Cis face (which faces the RER) - has medial section and then trans face which is opposite of RER which sandwhiches this  Vesicles at Cisterna tip fuse or pinch off  NO RIBOSOMES ON THEM – It’s smooth!  Three tpes of cisternae (cis, medial and trans) and two flanked networks of tubules: CGN faces RER and TGN is opposite to RER  Important for processing and sorting of proteins (particularly secreted, membrane, lysosomal proteins) Transport vesicles: budding and fusion  Pinching of the membrane – causing it to pucker o Have to get cargo inside of the vesicle o Soluble Cargo Protein – free floating in the ER o Membrane Cargo Protein - bound to the membrane of the ER  Cargo Receptor Protein – this will bind to soluble cargo proteins to make sure they’re incorporated in the newly formed vesicle  Coat proteins promote budding of vesicles o Bind to certain structures on the cytosolic face of the transmembrane protein o COPII Coat Protein (form COPII Vesicles)  RER to Golgi (Anterograde Transport) o COPI Coat Proteins (Form COPI Vesicles)  Cis-golgi to RER (Retrograde Transport) found on the target membrane (on the cis facing side of the golgi) o Clathrin vesicles  TGN/PM to late endosomes  GTP-Binding Proteins – control assembly and disassembly of COPII coat proteins o Membrane associated GTP binding proteins promote association of COPII coat proteins on ER membrane Lecture 5 – Where do Proteins go after ER?  ONCE COPII vesicles are released from donor membrane, hydrolysis of GTP to GDP which triggers disassemble of COPII coat proteins  GTP helps put the coat on, Hydroylsis of GTP to GDP (removing the GTP which acts as glue) takes the coat off  ATP DEPENDENT EFFECT  Snare proteins promote fusion of vesicles with target membranes (This is following formation of vesicles when it’s moved and reached the target membrane) o V-Snare – snare on the vesicle which combines with the T-Snare o T-Snare - Snare on the target membrane which wraps together with the V-snare to force the vesicle membrane and the target membrane to combine together RER to Cis-Golgi transport (ANTEROGRADE)  Taking soluble cargo and incorporating it in the vesicle  COPII Vesicles – Mediate anterograde transport o GTP Binding proteins control assembly and disassembly of COPII coat proteins and docking vesicles to target membrane  Cargo membrane proteins have sorting signals – sorting signals are recognized by COPII proteins o Initiating factor – soluble cargo containing DXE (Diacidic proteins) (ASP – x – Glu)  When you have soluble cargo protein which then binds to the membrane receptor can lead to the activation of COPII to trigger the association of the coat proteins to bind to that part of the membrane o The coat proteins will continue to do this until the vesicle forms and then it buds off  COP Vesicles mediate retrograde transport o This happens in a similar manner but using different coat proteins o Form by the same mechanism however – just different coat proteins and different designation (COPI vs COPII) o COPII USES COPII COAT PROTEINS Cystic Fibrosis  Recessive genetic diseases  Characterized by abnormal transport of chloride and sodium across epithelium leading to thick viscous secretions o Membrane associated protein which does the ion transport HAS TO GET TO THE MEMBRANE TO FUNCTION PROPERLY  Affects lung, liver, pancreas and intestine o Particular effect in lungs, inability to properly transport ions leads to detrimental osmotic affect which leads to thick, viscous mucous which can’t be cleared – difficulty for breathing and petri dish for bacteria, leads to terrible lung infections Lecture 5 – Where do Proteins go after ER?  Caused by a mutation in the gene for the protein cystic fibrosis transmembrane conductance regulator (CFTR)  ΔF508 is most common mutation in CFTR o This mutation leads to an inability to move the membrane transport protein to the membrane! o Normally the protein would be transported by COPII to Golgi and then via Secretory vesicles to the membrane of the cell, but with this mutation interferes with COPII coat proteins ability to bind to the ER membrane o CFTR has a di-acidic (DXE) sorting signal which interferes with the exposure of that signal to the ER membrane FACE o With that mutation Di-acidic signal of CFTR doesn’t “stick out” and
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