BIO250 - Final Exam - Lecture 3 Cell Junctions -occur at...

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Lecture 3 Cell Junctions -occur at points of cell-cell or cell-matrix contact -3 different groups with 3 different functions: -Anchoring junctions: mechanically attach cells to other cells or to the extracellular matrix -Occluding junctions: seal the contacts between cells and prevent the movement of particles between cells (think of “occlusion”) -Communicating junctions: allow chemical or electrical signals to pass between cells -the challenge for cell junctions that mediate cell adhesion is how to resist the forces generated by the relatively massive cells they are supposed to hold together -adhesion proteins are tiny compared to the cells they are holding together ** cell adhesion complexes display two features to overcome this challenge -clustering adhesion receptors increases the strength of the adhesion proteins through their additive effect/through avidity = strength in numbers, like Velcro – the pulling forces are distributed among many proteins -clustered receptors link cytoskeletal networks = produce huge tension-bearing protein interaction network that runs through the tissue – now the cells aren’t just linked and pulling against adhesion proteins but against each other’s cytoskeleton network -affinity = binding strength between two molecules via a single binding site -avidity = the total binding strength between two molecules or complexes involving multiple binding sites *avidity is higher than the sum of the individual binding affinities of the interactions involved because dissociation requires that all the interactions be broken at the same time = co-operative binding – also, the pulling forces are distributed among many binding proteins therefore each one is exposed to less force than it would be if it was alone (think of Velcro) Cytoskeletal Attachments - Actin and Microtubule Attachment sites: Adherens junctions = cell-cell Focal Adhesions = cell-matrix - Intermediate Filament Attachment sites: Desmosomes = cell-cell Hemidesmosomes = cell-matrix ** in both cases, cell-cell interactions use cadherins (because you want to “adhere” them together) and cell-matrix use integrins (because you just want to “integrate” parts of them together)
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Adherens Junction Structure: -cadherins = TM proteins that allow signals to pass between cells and are part of the family of calcium-dependent cell-cell adhesion molecules in animal cells -adherens junctions can form strong continuous adhesion belts = zonula adherens across many cells -adherens junctions are critical for adhering cells to form epithelia – the sheets of cells that line our body compartments - **these junctions are found closer to the apical side of the cells (closer to the lumen –basolateral is closer to the bloodstream) -adherens junctions are formed by clustered cadherin adhesion receptors: -actin filaments in one cell bind to anchor proteins (composed of catenin/plakoglobin, actinin, vinculin) – the intracellular tail of cadherin binds to the anchor protein and the cadherin is TM so part of the cadherin crosses the plasma
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BIO250 - Final Exam - Lecture 3 Cell Junctions -occur at...

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