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341lecture25nov9sakai - Cell Biology 341 Last lecture...

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Cell Biology 341 Monday, November 9, 2009 Today: Two more types of single-pass membrane proteins Synthesis of multipass membrane proteins Chapter 13: Golgi complex and vesicle traffic in the secretory pathway Chapter 13 reading: skim oligosaccharide chemistry pages 774- 776, omit pages 795- 809 Last lecture: Mitochondrial proteins receptor/protein translocators N- terminal signal cleaved in matrix Proteins directed to the nucleus or mitochondria completed synthesis in the cytosol before sorting Proteins that begin synthesis in cytosol completion on rough ER: SRP ER signal sequence SRP receptor in the ER translocation into the lumen the N-terminal signal is cleaved soluble proteins ER lumen Some single-span membrane proteins contain both an N-terminal cleaved signal and an internal stop-transfer sequence ( C-terminal in cytosol)
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Other single-pass transmembrane proteins contain internal start-transfer sequences The start-transfer sequence is bound by the SRP, and opens the translocator pore just like an N-terminal sequence The ribosome finishes synthesis of the protein The hydrophobic start-transfer sequence slips through the lateral gate to become a transmembrane α helix If the + charge is on the N- terminal side of the hydrophobic sequence, the N- terminal of the protein will be on the cytosolic side If the + charge is on the C- terminal side of the hydrophobic sequence, the C- terminal will be in the cytosol The + charge faces the cytosol Figure 12-47 SRP, receptor and ribosome not shown
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