Lecture18.2010 - Bio 320: Cell biology Lecture 18: March...

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Bio 320: Cell biology Lecture 18: March 30,2010 Lecture 18: Extracellular Matrix, Integrins, Cell-Substrate adhesion Reading: Alberts: Chapter 19: pg 1164-1194 Mediating Intercellular Communication: Gap Junctions I. Gap junctions - plasma membranes of adjacent membranes come very close together (~3 nm) but no direct contact; sites between animal cells specialized for intercellular communication A. Gap spanned by very fine strands - form molecular pipeline connecting adjacent cell cytoplasms II. Gap junctions composed of integral membrane protein connexin (multigene family; •12 reported) A. Connexins cluster to form multisubunit complex called connexon; completely spans membrane 1. 6 subunits around central opening (annulus; ~1.5 nm diameter) 3. Some cells make >1 connexin; don't know if single connexon contains >1 connexin B. Connexons join through interactions of connexin subunit extracellular domains forming channel connecting cell cytoplasms once aligned 1. Connexons cluster in specific regions forming plaques; visualized by freeze-fracture C. Connexins have distinct tissue-specific distributions 1. Connexons made of variant connexins -> variations in conductance, permeability, regulation 2. Sometimes, connexons in neighboring cells made of different connexins can dock & form functional channels, other times they can’t 3. Compatibility differences may play important roles in promoting or preventing cell communication in organs D. Connexons joining cardiac muscle cells made of connexin Cx43; those joining cells of heart’s electrical conduction system made of Cx40 - they are incompatible, can’t form working channels 1. The cells are electrically insulated from each other even though they are in physical contact III. Gap junctions mediate gap junction i ntercellular c ommunication (GJIC) A. Ionic currents & low MW dyes (fluorescein; £ ~1000 daltons) pass through gap junctions 1. Nonselective; if small enough molecule passes through pipeline; also thought to be gated B. Several treatments can cause closure of gap junctions, including elevated intracellular [Ca 2+ ] 1. Inject Ca 2+ ions -> gap junctions close 2. Main control method is probably phosphorylation of connexin subunits by protein kinase IV. Cardiac & smooth muscle - contraction of mammalian heart stimulated by electrical impulse generated in small region of specialized heart muscle (sinoatrial node); acts as pacemaker 1
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A. Impulse spreads rapidly as ion flow from 1 cardiac cell to next via gap junctions; cells contract B. Intestine/esophagus smooth muscle peristaltic waves due to current flow through gap
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Lecture18.2010 - Bio 320: Cell biology Lecture 18: March...

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