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Transcript
MCB Lecture 13 – Actin What is G-actin? o Globular Actin, which assembles to form F-Actin What is F-actin? o Fibrous Actin, made from G-actin combining There are two types of filaments made from F-actin. What are they? o Microfilaments o Thin Filaments What are the important functions of Actin? o Cell Surface Shape o Muscle Contraction Which end of Actin grows to form the filament? o Both. o – End is slow growing o + End is fast growing What happens when a high concentration of ATP is bound to G-Actin? o Monomers add to both ends, but add faster at the + end. o The Hydrolysis of ATP is slow. What happens when a low concentration of ATP is bound to G-Actin? o Treadmilling can occur if it is not too low. This is where monomers add to + end and come off of the – end at the same rate, causing the length of the filament to remain the same as it moves. o Too much can lead to dissociation. Where is the highest density of actin filaments located? o At the cell periphery In Actin Nucleation, proteins are needed to elongate the strands. There are three main proteins. What are the names of these proteins, and what do they do? o Arp2/3: Stimulates nucleation of G-Actin into dimers/trimmers (initiates polymerization) o Formins: Nucleate growth of straight, unbranched actin filaments o Profilin: Exchanges ADP for ATP, so monomers can continue attaching. What complex attaches to create a branched actin network? o ARP Complex There are four proteins that modify filament elongation. Given each protein name, state what is does: o Thymosin? Binds G-Actin Prevents F-Actin Assembly o Profilin? Competes with Thymosin Binds G-Actin Promotes filament assembly o Cofilin? Promotes disassembly at the – end o Gelsolin? Caps + end to prevent further growth What is the difference between Lamellipodia and Filipodia? o Lamellipodia push the cell forwards for movement o Filipodia act as sensors that respond to chemotaxic factors What type of Myosin is used for Contractile Muscles? o Type II Myosin Actin is organized differently in the cell depending on if it is a Contractile Bundle (Stress Fiber), it is Lamellipodia, or it is Filipodia. How are the fibers arranged in these three? o Contractile Bundle: Straight, loosely packed, anti-parallel o Lamellipodia: Random, loose o Filipodia: Tightly packed and parallel What three cross-linking proteins form parallel cables/ bundles? o A-Actin o Fimbrin o Villin What two cross-linking proteins form webs/gels? o Spectrin o Filamin Which Actin Protein is contractile? o A-Actin Which Actin Protein is non-contractile? o Fimbrin Answer the following questions about Stress Fibers and Focal Adhesions: o What are Focal Adhesions used for generally? Cell movement o What are Stress Fibers? Contractile bundles of actin filaments inside of the cell o What are Focal Adhesions? A clustering of integrin receptors at the membrane o Where to the integrin receptors bind? The extracellular matrix o What do integrin receptors bind to on the inside of the cell? Adapter Proteins (Vinculin & Talin) Stress Fibers o What promotes the disassembly of Focal Adhesions at the back end as the cell moves forward? Cofilin o What are the stress fibers cross-linked by? A-Actin (contractile) o What anchors the actin cytoskeleton to the Sarcolemma in Skeletal Muscles? Dystrophin What type of junction is formed by actin? What protein is involved? o Adherens (tight) junctions o Cadherin is involved. Microvilli and Stereocilia are made of actin. What two linking proteins are involved? o Fimbrin o Villin Filamin and Spectrin are actin filaments that make webs for support of the cell. Which causes 2D crosslinks and which causes 3D crosslinks? o Filamin: 3D o Spectrin: 2D Which G-Protein is used in Actin Polymerization? o Rho What are the three different type of Rho? o RAC o CDC42 o RHO Which Rho interacts with each cytoskeleton protein? o RAC: Lamellipodia o CDC42: Filipodia o RHO: Stress Fibers