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The Cytoskeleton (Major Proteins (Actin (Present in all eukaryotes and…
The Cytoskeleton
Major Proteins
Actin
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Polymerization: Depends on K, Mg, and ATP. Three phases = nucleation, elongation, and steady state. Can be visualized using fluorescence microscopy.
Actin polymerization is regulated by actin binding protein. e.g. Arp2/3 complex, formins, capping protein, cofilin, and alpha-actinin. G actin monomers form F actin filaments
Organized into structurally distinct networks in cells. Assembly is accomplished by large number of actin binding proteins
Capping Protein, Cap Z regulates actin length. Blocks elongation and stabilizes the plus end.
Actin nucleation is accomplished by Arp2/3 complex and formins. Formins bind to dimeric complex and remain bound to the plus end. Nucleates linear actin filaments NO branches.
Wiskott-Aldrich Syndrom/WASP: X-linked recessive, characterized by eczema and thrombocytopenia. Mutation in WASP gene. WASP is an actin-binding adaptor proteins that function as nucleation promoting factors. WASP is autoinhibited. NO C terminal and N terminal interactions.
Dendritic nucleation model: Multiple actin binding protein cooperate to form and turnover dendritic networks of actin filaments.
Actin drives cell movememnt. Establishes traction and contraction at the rear of cells. Cell moves forward. e.g. Listeria monocytogenes.
Actin cross links proteins, e.g. microvillus.
Intermediate Filamennts
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Elongated proteins with a central alpha helical domain and promotes dimerization. NO binding to nucleotides, NO polarity, easily bent but difficult to break, very stable.
Examples: keratin (epithelial cells), neurofilamines (axons), vimentin-like filaments (Muscle), and nuclear lamins (nucleus)
Tubulin
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tubulin = heterodimer of alpha and beta tubulin. Each can bind GTP. Only Beta tubulin bings to GDP and GTP.
Hollow, stiff cylinder from 13 parallel protofilaments. Polar filaments similar to actin. alpha at the minus end, beta at the plus end. Grows faster on the plus end.
Dynamics regulated by GTP hydrolysis. GTP bound= polymerize, GDP bound= depolymerize. Growing microtubule has a GTP cap. Loss of cap leads to a catastrophe; microtubule shrinks.
Dynamic instability= rapid conversion between growth and shrinkage.
Nucleation occurs at the microtubule-organizing center (MTOC). Used as a template to develop microtubules.
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Dynamics controlled microtubule binding protein. Associate with motor proteins (kinesin= walk towards the plus end, dynenin = walk toward the minus end) to transport cargo and move organelles. Flagella and cilia are made of mictroubules and dynein. Core = axoneme, with 9+2 arrangement
Organizes the cell internally, controls shape, regulates stiffness and tension, drives movment/attachment, guides transport of proteins, splits cells/moves chromosomes, generates forces.