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Lecture 6

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University of Toronto St. George
Jane Mitchell

Lecture 6 Intracellular Compartments and Protein SortingMany proteins need to enter ER for modification w sugarsThis occurs at same time they are being synthesized by ribosomesTranslation begins w synthesis of short signal peptide sequence a signal recognition particle protein complex binds to peptide which translation continuesSRP then binds to receptor in ER membrane anchoring ribosome which binds its receptor and the signal peptide meets the translocatorTranslation continues and protein passes through translocatorThe signal peptidase cleaves the signal peptide leaving the new protein molecule in lumen of ERSignal sequences are a stretch of AA seq of protein that directs protein to correct location in cellIn gene located at beginning5 end Found at N terminus of proteinA signal peptidase can remove signal from proteinSignal sequences can be internal stretches of AA which remain part of proteinSorting of a secreted protein o Translation starts on cytosolic ribosomes o Signal sequence at amino terminal end directs protein to ER o The signal sequence is hydrophobicit interacts w translocatorAfter it is cleaved it ends up in membrane of ER which is then degraded by other proteinsThis is known as cotranslational translocation to ER bc it happens while the protein is still being translated o Protein is inserted through membrane by a protein translocator o Signal sequence cleaved left being in ER membrane secreted protein ends up in ER lumen o Secreted protein moves in transport vesicles via secretory pathway o Released by exocytosis at the plasma membraneThe vesicle fuses with plasma membrane secretionTransmembrane proteins do not get secreted you need it to stay part of membraneOnce its been synthesized it stays in membrane and it is transferred via vesiclesSome proteins will eventually end up as membrane spanning protein Transmembrane proteinStart transfer sequence targets protein to ERwhich pass through translocator and it binds interacts w translocator protein and protein continues to be translatedStop transfer sequence hydrophobicBinds hydrophobic that stretch of 2030 AA which tells translocator is stop sequencing Other parts are of protein are hydrophilic translocator opens in diff mannercan open sideways and release content in membrane mature protein and start seq
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