965-1025 - The cytoskeleton THE SELF-ASSEMBLY AND DYNAMIC...

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The cytoskeleton THE SELF-ASSEMBLY AND DYNAMIC STRUCTURE OF CYTOSKELETAL FILAMENTS Intermediate filaments provide mechanical strength . Microtubules determine the positions of membrane enclosed organelles and direct intracellular transport . Actin filaments determine the shape of the cell’s surface and are necessary for whole-cell locomotion . Accessory proteins are essential. Cytoskeletal filaments are dynamic and adaptable o Alteration or structural rearrangement in a cell requires little energy when conditions change o Microtubules can rearrange themselves to form a bipolar mitotic spindle during cell division o Microtubules can form whips called cilia and flagella o Actin filaments support the plasma membrane o Actin filaments also form surface projections o Intermediate filaments line the inner face of the nuclear envelope forming a protective cage for DNA o Intermediate filaments are twisted into strong cables o Actin and its motor protein myosin form a belt around the middle of the cell (contractile ring) pinches the cell into two The cytoskeleton can also form stable structures o Important in cell polarity o 2 types of actin assemblies: actin cables and actin patches Each type of cytoskeletal filament is constructed from smaller protein subunits o Intermediate filaments are made up of subunits that are elongated and fibrous o Actin filaments and microtubules are made of subunits that are small and round o Covalent linkages between their subunits hold together many biological polyemrs o Weak noncovalent interactions hold together the 3 types of cytoskeletal polymers o Accessory proteins bring cytoskeletal structure under the control of extracellular and intracellular signals
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Filaments formed from multiple protofilaments have advantageous properties o Cytoskeletal polymers are made out of multiple protofilaments o Most cytoskeletal filaments to resist thermal breakage, while leaving the filament ends as dynamic structures at which addition and loss of subunits can occur rapidly o Many hydrophobic and weak noncovalent bonds hold the subunits in a cytoskeletal filament together o Intermediate filaments assemble by forming strong lateral contacts between alpha- helical coiled coils o Intermediate filaments tolerate stretching and bending because the subunits are staggered in the filament o Microtubules subunit bonds are weak. They break much more easily when they are bent than do intermediate filaments o Single protofilaments are thermally unstable o Multiple protofilaments are thermally stable Nucleation is the rate-limiting step in the formation of a cytoskeletal polymer o For a large filament to form , subunits have to come together into an initial aggregate or nucleus, o Initial process of nucleus assembly is called filament nucleation o Concentration of free subunits left in solution at this point is called the critical concentration, Cc The tubulin and actin subunits assemble head-to-tail to create polar filaments
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965-1025 - The cytoskeleton THE SELF-ASSEMBLY AND DYNAMIC...

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