Ch_12_Cytoskeleton_and_movement - 3 types of filaments 1...

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3 types of filaments 1. Microfilaments (actin filaments)- cell movement 2. Intermediate filaments- structural support 3. Microtubules- hollow, used to grab chromosomes during cell division, serve as highways for protein movement ACTIN Major cyskeletal protein of most cells Polymerizes to form actin filaments (microfilaments) o Each monomer rotated by 166 degrees. o 13.5 actins to see a repeat from one actin. o Have distinct polarity at ends (barbed and pointed) About 7nm in diameter All actins are similar in AA sequence and are highly conserved Monomeric actin becomes a dimer, which easily dissociates Proteins interact with cell exterior to stimulate dimerization Dimers can become trimers, which is called nucleation. Nucleation is first step in actin polymerization. consists of 3 monomers filaments grow by addition of monomers to both ends barbed end grows faster than pointed end o ATP bound actin polymerizes more readily than ADP bound actin. Rate at which actin incorporated into filaments is proportional to concentration. Treadmilling: dynamic behavior of actin filaments o Net loss of monomers from pointed end balanced by addition to barbed end. o Treadmilling requires ATP, with ATP/actin adding to barbed end, while ADP/actin dissociating from pointed end. o Treadmilling is important in the formation of cell process and in cell movement Drugs can affect actin polymerization o Cytochalasins bind to barbed end and prevent elongation o Phalloidins bind to sides of actin filaments and prevents ADP actin from dissociating. o Gelsolin affects actin in different ways depending on how the system is manipulated. Actin binding proteins regulate formation and stability of actin cytoskeleton. (eg. Formin, Arp2/3, ADF/cofilin, profilin) Nucleation is rate-limiting step in filament formation. 2 proteins determine where filaments are formed within the cell by nucleating actin filaments:
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1. Formin- nucleate long unbranched actin filaments that make up stress fibers and thin filaments of muscle cells. o Each subunit of a formin dimer binds an actin monomer. (dimerizes actin), then a third monomer binds, followed by rapid polymerization. o Formin attached to BARBED end of filament. 2. Arp2/3 complex o Binds filaments near the barbed end and initiates branching at the leading edges of cell. o 7 molecule complex (2 large subunits, 5 small subunits) o Promotes nucleation to cause branching o Direction is determined chemically, “follows scent of nutrients” Actin pushes membrane in the direction of stimulus. 3. ADF/Cofilin o Preferentially binds to actin/ADP monomers. o Binds to actin filaments and enhances rate of dissocation from pointed end. o
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This note was uploaded on 03/19/2008 for the course MMG 409 taught by Professor Arvidson during the Spring '08 term at Michigan State University.

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Ch_12_Cytoskeleton_and_movement - 3 types of filaments 1...

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