Lecture 7, exocytosis and endocytosis.pptx - Membrane...

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Membrane trafficking at the plasma membrane: ExocytosisEndocytosis
Figure 13-1 Molecular Biology of the Cell(© Garland Science 2015)To keep the surface area of the plasma membrane roughly constant through time, these two processes must be balanced.cell exterior
Two basic types of exocytosisFigure 13-62 Molecular Biology of the Cell(© Garland Science 2015)NO further signal is needed for proteins in the lumen of the TGN to enter the constitutive secretory pathway (default pathway). only in cells specialized for secreting certain products on demand (e.g., neurons → neuro-transmitter; -pancreatic cells → insulin; mast cells → histamine)in all cells(e.g., acetylcholine, insulin, histamine)
Figure 13-5 Molecular Biology of the Cell(© Garland Science 2015)At the TGN, no known coat proteins function in the formation of vesicles in the constitutive and regulated secretory pathways.lysosomeconstitutive secretoryregulated secretory
Figure 13-64A Molecular Biology of the Cell(© Garland Science 2015)In the TGN, not well understood signal patches on certain proteins may direct selectiveaggregation of these proteins, leading to their sorting into regulated secretory vesicles, which undergo fusion as part of the maturation process.Acidification of immature regulated secretory vesicle by H+pump (H+ATPase) on vesicle membrane contributes to further aggregation of proteins.Retrieval of membrane that is recycled back to (late) endosome and the Golgi further contributes to concentration of protein aggregates in these vesicles.XX
Figure 13-66 Molecular Biology of the Cell(© Garland Science 2015)Precursors of secretory proteins (pro-proteins) are often proteolytically processed by specific proteases during the formation/maturation of regulated secretory vesicles. DON’T MEMORIZE THISfor ER translocationHowever, NOT ALL proteins that are secreted in the regulated secretory pathway need to undergo processing in the immature secretory vesicles.For example:
For some regulated secretory vesicles, the molecules to be secreted are NOT proteins.For example, the neurotransmitter acetylcholine is a relatively small molecule that is synthesized in the cytosol and transported by specific transporters into synaptic vesicles (a type of regulated secretory vesicle found at the pre-synapse of some neurons).
Figure 13-65 Molecular Biology of the Cell(© Garland Science 2015)Like other membrane fusion events, Rab GTPases and SNAREs are involved in the tethering, docking and fusion steps at the plasma membraneabIn the regulated secretory pathway, one of these steps may be regulated in a variety of ways (e.g., see next slide)CYTOSOLTETHERING and
For example, at the pre-synapse of a resting neuron, synaptic vesicles are already docked at the pre-synaptic plasma membrane.Figure 13-67B Molecular Biology of the Cell(© Garland Science 2015)synaptotagminAND RELEASE OF NEUROTRANSMITTERsynaptic vesicleMembrane fusion does not occur because the protein complexin binds to and prevents complete assembly of the trans-SNARE

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