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Ensure vesicles fuse only with the correct membraneSNAREs - transmembrane proteins important for docking the vesicle in place SNAREs on the vesicle (v-SNAREs) interact with complementary SNAREs on the target membrane (t-SNAREs)Vesicle Fusion - deliver its cargo and adds vesicle membrane to organelle Vesicle Reaches Target - Recognize and Dock with the Organelle
* Fusion requires the two lipid bilayers come within 1.5nm of each other so lipid can intermix* Need to displace water from hydrophilic surface of the membrane***After pairing the v-SNAREs and t-SNAREs wrap around each other = pulls two membranes into close proximityMembrane fusion sometimes needs a specific molecular signal
How do proteins cross the plasma membrane? Secretory Pathways
•Exocytosis - newly made proteins, lipids, and carbohydrates are delivered from the ER, via the Golgi apparatus, to the cell surface by transport vesicles that fuse with the plasma membrane•FIXED sequence of membrane-enclosed compartments - often chemically modified en routeExocytosis
•Disulfide bonds are formed between cysteine side chains Remember?Stabilize protein structure•Glycosylation - covalent attachment of short oligosaccharide side chains Remember? Protect extracellular proteins, form carbohydrate layer, help with cell to cell recognition–Carried out by glycosylating enzymes in ER but not in the cytosolChemical modifications which occur in the ER