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Intracellular Compartments and Transport •Membrane Enclosed Organelles •Protein Sorting •Vesicular Transport •Secretory Pathways •Endocytic Pathways •http://aimediaserver.com/studiodaily/videoplayer/?src=harvard/harvard.swf&width=640&height=520 Copyright © 2005 Pearson Prentice Hall, Inc. Intracellular Compartments and Transport: Vesicular Transport Function: carry soluble proteins & membrane between compartments Budding: driven by assembly of a protein coat Docking: specificity depends on SNARES Copyright © 2005 Pearson Prentice Hall, Inc. VESICULAR TRANSPORT • Vesicles bud off a compartment • Transport between compartments – Secretory Pathway: Golgi to plasma membrane – Endocytic Pathway: plasma membrane to endosome to lysosome Copyright © 2005 Pearson Prentice Hall, Inc. Copyright © 2005 Pearson Prentice Hall, Inc. VESICULAR TRANSPORT 1. Vesicles bud off a compartment 2. Transport between compartments Endocytic Pathway: plasma membrane to endosome to lysosome Secretory Pathway: Golgi to plasma membrane Copyright © 2005 Pearson Prentice Hall, Inc. Vesicle Budding • Protein coat assembly – on cytosolic surface – COP- coated • ER to Golgi • Within Golgi – Clathrin coated • • • • Clathrin coated pit Dynamin protein : constriction, binds GTP Adaptin proteins: anchor clathrin, cargo receptors Cargo receptor : bind transport signals Copyright © 2005 Pearson Prentice Hall, Inc. Clathrin: Clathrin Coated Pit => Clatherin Coated Vesicle Copyright © 2005 Pearson Prentice Hall, Inc. Vesicle Budding: Clathrin Clathrin coated pit Dynamin protein : constriction, binds GTP Adaptin proteins: anchor clathrin, cargo receptors Cargo receptor : bind transport signals Copyright © 2005 Pearson Prentice Hall, Inc. Vesicle Docking • Organelle Recognition • v-SNARE • t-SNARE • Membrane Fusion – Water removed: enzyme complex – Intertwining of SNAREs Copyright © 2005 Pearson Prentice Hall, Inc. Copyright © 2005 Pearson Prentice Hall, Inc. Endocytic Pathways 1. Phagocytosis: Specialized Phagocytic Cells Ingest Large Particles 2. Pinocytosis: Take up Fluids and Macomolecdules 3. Receptor Mediated Endocytosis: Forms A Specific Route into Animal Cells 4. Endsomes: Sorting Endocytosed Macromolecules 5. Lysosomes: Principal Site of Intracellular Digestion Copyright © 2005 Pearson Prentice Hall, Inc. Endocytic Pathway: Phagocytosis, Pinocytosis • Phagocytosis : phagocytic cells (macrophages) – Bacteria, dead or damaged cells, deris – Pseudopods form phagosome – Phagosomes fuse with lysosomes • YB bacteria : interfere with fusion – Recognition molecules : phagocytic receptors • Opsonised bacteria Copyright © 2005 Pearson Prentice Hall, Inc. Copyright © 2005 Pearson Prentice Hall, Inc. Endocytic Pathway: • Pinocytosis – Fluid and small particles – By clathrin –coated pits – Fluid intake balanced by fluid exocytosis Copyright © 2005 Pearson Prentice Hall, Inc. Receptor Mediated Endocytosis: Cholesterol Uptake • Cholesterol transported as LDL • Endocytosed via receptors for LDL • LDL-receptor complex dissociated in endosomes – Receptor recycled to membrane – LDL to lysosome • Protein –cholesterol dissociated • Cholesterol released to cytosol • Genetic condition: defective LDL receptor Copyright © 2005 Pearson Prentice Hall, Inc. Receptor Mediated Endocytosis: Cholesterol Uptake Copyright © 2005 Pearson Prentice Hall, Inc. Fate of Receptors in Endosomes Recycled Degraded Transcytosed Copyright © 2005 Pearson Prentice Hall, Inc. • • • • Lysosomes Sacs of hydrolytic enzymes pH ~ 5 : optimal activity of enzymes Enzymes: tagged with mannose-6-P in ER Unique membrane – ATP proton pump – Transport proteins – Membrane proteins glycosylated Copyright © 2005 Pearson Prentice Hall, Inc. Protein Glycosylation Copyright © 2005 Pearson Prentice Hall, Inc. Secretory Pathways Covalent Modification in ER ER Exit Controlled to Ensure Protein Quality Further Modification & Sorting in Golgi Released from Cell by Exocytosis Copyright © 2005 Pearson Prentice Hall, Inc. Secretory Pathways Constitutive Regulated Copyright © 2005 Pearson Prentice Hall, Inc.
