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Secretion, Exocytosis & Endocytosis. Lecture 22 BSCI 420,421 Oct 23,24, 2002 An old Cherokee told his grandson about a battle going on within him. “It is between two wolves, one is evil, full of anger, greed, envy, pride, and ego. The other is good, full
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Secretion, Exocytosis & Endocytosis Lecture 22 BSCI 420,421 Oct 23,24, 2002 An old Cherokee told his grandson about a battle going on within him. “It is between two wolves, one is evil, full of anger, greed, envy, pride, and ego. The other is good, full of generosity, kindness, empathy, love, and humility.” “Which wolf wins?” said the grandson. The old Cherokee replied, “The one I feed.”
In addition to lysosomes, two secretory paths diverge from TGN
Constiutive pathway • Continuous • COP-1 coated ves from TGN • No stimulus required for exocytosis • Provides new plasma membrane proteins • Continuously secreted proteins, e.g.: • albumen from liver • Igs from plasma cells • Collagen from fibroblasts • Default pathway that requires no additional signal
PM proteins can be targeted to 2 different domains, directly or indirectly
Lipid rafts are often transported to the cell surface as a group. Rich in cholesterol, long chain phospholipids & glycolipids, & Proteins w long TM domains.
Regulated secretion • Specific receptors bind proteins and cluster into • clathrin coated areas of the TGN. • Proprotein cleaved to mature protein • Ions pumped out and H20 follows to concentrate • Stored in secretory granule until stimulus • Transported by MTs upon stimulus • Docking, Fusion, & exocytosis, often stim.by Ca2. • E.g.s: protein hormones, inducers, cytokines, digestive enzymes
Formation of Secretory Vesicles and Maturation Maturation involves loss of clathrin coats, dehydration and concentration, and proteolysis. (pre-proinsulin -> proinsulin -> Insulin) Gold-labeled Anti-clathrin
Transport (MTs), docking, and fusion. Stimulated by receptor activation that increases Ca+2 in cells. Fig 57: Insulin secretion by exocytosis from pancreatic Beta cell.
In addition to 1) Exocytosis, three other types of secretion: 2) Apocrine secretion Release of a part of the cytoplasm, surrounded by pl. membrane. Egs: platelets from megakaryocytes, Milk fat globules from mammary gland cells 3) Holocrine secretion Release of complete cells into & out from a secretory duct Egs: Eccrine sweat gland cells- keep skin moist and oiled. Spermatozoa? 4) Monomolecular secretion Proteins secreted one molecule at a time w no membrane Egs: Bacterial secretion, yeast mating pheromones, and other ABC transporter secreted proteins
ENDOCYTOSIS – Uptake of macromolecules into cell in vesicles 1. Phagocytosis 2. Pinocytosis /Receptor-Mediated endocytosis Ingestion of large particles Activated by receptors on surface e.g. Fc rec for antibodies on WBCs like neutrophils: Fig 40 Triggers actin & myosin filament assembly & interaction to form pseudopods & engulf particles.
2. Pinocytosis /Receptor-Mediated endocytosis (RME) • Formation of clathrin-coated pits • RME was discovered by Brown & Goldstein by looking at skin • Fibroblasts of children with Familial Hypercholesterolemia, a gene • disease in which excess cholesterol causes early heart attacks. • Mutant LDL receptors, so unable to take LDL out of bloodstream. • LDL particle • ApoB protein ~1500 • cholesterol • esters
RME of LDL particles Can also be defective LDL binding Or defective targeting of receptor to the cell surface
Transcytosis- Eg newborn takes up antibodies from mother’s milk (Colostum) release at neutral pH Uptake at acid pH
