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55-140 (56)
Habetler (53)
Lecture

EXOCYTOSIS

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Department
Biological Sciences
Course
55-140
Professor
Habetler
Semester
Fall

Description
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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