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55-140 (56)
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Biological Sciences

EXOCYTOSIS Signals on proteins are the key to targeting Bound ribosomes make ALL secretory and membrane proteins Pulse chase to figure out vectorial transport (in pancreas) Vesicular Transport: reduces need to cross membranes allows for compartmentalization problems: targeting maintaining compartmental identity (keep certain proteins/membranes in a given compartment) ER: cytoplasm-filling reticular network prominent in secretory cells and steroid metabolizing cells continuous with nuclear envelope MORPHOLOGY - cisternae = flat sacs that makeup the ER 50% of total cell membrane lumen: separate secretory from cytoplasmic proteins prepare proteins for export ion storage FUNCTIONS - lipid synth, ion storage and trans, steroid metabolism RER only: export of proteins to secretion, membrane, lysosome, endosome ER can be isolated by centrifugation BLOBEL – “signal hypothesis” signal sequence to translocate through membrane and is later cleaved sorting begins at the ribosome (moves to ER during translation) SRP – signal recognition particle **adaptor molecule** free floating and binds signal sequence recognized by and binds to ER membrane receptor causes a pause in translation until the pore is reached translation is co-translational (otherwise signal sequence is hidden) SIGNAL SEQ – not highly conserved general features: hydrophobic core cleavage site – small hydrophobics **not all are cleaved** SEC61 TRANSLOCON – forms pore tightly regulated – mech. unknown pore lines up perfectly with ribosome exit tunnel ASSESSING TRANSLOCATION EXPRIMENTALLY: the membrane is impermeable to trypsin find that translocated proteins are proteolytically cleaved can also see co-translation INTEGRAL MEMBRANE PROTEIN TRANLOCATION “stop transfer sequences” – stop the translocation but not translation hydrophobic sequences that stick into the membrane channel exit mech. still uncertain signal sequence orientation determines whether N or C terminus is outside many start-stops  polytopic membrane proteins ER PROTEIN PROCESSING: N-linked glycosylation occurs co-translationally inside the lumen assists in folding increases size of proteins (on experimental gels discussed above) chaperone-assisted folding disulfide bond formation (lumen is an oxidizing environement) GOLGI COMPLEX: polarized stack of cisternae (cis to trans) held together by tubules vesicular transport between compartments membrane budding/fusion easier than membrane translocation membrane topology is always conserved (cytosolic stay cytosolic) LUMEN is therefore topological equivalent of cell EXTERIOR steps of vesicular transport: 1. budding 2. targeting 3. fusion involve lots of machinery coat proteins, targeting proteins, regulatory proteins BUDDING: G-protein
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