haun72 - Medical Biochemistry Membranes: Bilayer...

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Medical Biochemistry Membranes: Bilayer Properties, Transport Lecture 72
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Synthesis of secretory proteins - review 1. N-terminal signal sequence is synthesized 2. Signal bound by SRP, complex docks with SRP receptor on ER membrane 3. Signal sequence binds to translocon, internal channel opens, inserted into translocon 4. Polypeptide elongates, signal sequence cleaved 5. ER chaperones prevent faulty folding, carbohydrates added to specific residues 6. Ribosomes released, recycle 7. C-terminus of protein drawn into ER lumen, translocon gate shuts, protein assumes final conformation
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Synthesis of integral membrane protein Integral membrane protein may, or may not have N-terminal signal sequence In absence of N-terminal signal sequence, internal signal sequence bound by SRP Animation:ERimport.mov SRP-protein-ribosome complex docks with SRP receptor, C-terminal portion of protein cotranslationally inserted into lumen of ER Mature protein transverses ER bilayer forming integral membrane protein NOTE: Orientation of protein within membrane dependent upon cluster of charged residues adjacent to internal signal sequence In presence of N-terminal signal sequence, integral membrane protein produced by stop-transfer signal that forms transmembrane domain
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Secretory Pathway Once a protein has entered exocytotic pathway, in general, it never returns to cytosol (notable exception is misfolded proteins - retrograde transport for degradation) In the absence of a sorting signal, protein will follow constitutive secretory pathway ( i.e., directed to plasma membrane) in transport vesicles Some proteins contain retention signals ( e.g., KDEL in C-terminus of some ER proteins)
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Secretory Pathway In specialized cells, regulated secretory pathway leads to packaging of product in secretory vesicles
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Asymmetry of proteins and lipids maintained during membrane assembly Orientation of a protein (asymmetry) is determined upon entry into ER, does not change during transit to other membrane/organelle Fusion of a vesicle with the plasma membrane preserves the orientation of any integral proteins embedded in the vesicle bilayer Animation: Secretion.mov
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Small GTPases Act as Molecular Switches GDP GTP GTP GDP “Inactive” “Active” GEF P i GAP GTP exchange for bound GDP, facilitated by Guanine-nucleotide Exchange Factors (GEFs), “activates” protein (usually resulting in conformational change). Hydrolysis of GTP GDP, accelerated by GTPase-Activating Proteins (GAPs), “inactivates” complex. ARF - vesicular transport Ran - nuclear transport Rab - regulated secretion, endocytosis, intracellular transport Rho - formation of actin cytoskeleton Ras - growth and differentiation signaling pathways
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Intracellular Transport Vesicles Step 1: Coat assembly initiated Step 2: ARF recruits coat proteins Step 3: Vesicle budding Step 4: Coat disassembly Step 5: Vesicle targeting (v- SNARE) Step 6: General fusion machinery assembles (NSF, SNAP)
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haun72 - Medical Biochemistry Membranes: Bilayer...

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