Filaments Of The Cytoskeleton

• Actin Filaments

• Microtubules

• Intermediate filaments

– Helical polymers of protein actin

– Hollow cylinders of protein tubulin

– Ropelike fibers of inter-mediate filament proteins

• Tubulin subunits are heterodimers of -tubulin and -tubulin• GTP bound to tubulin• Hollow cylinder of 13 parallel protofilaments• Tubulin subunits in same orientation

Microtubule Structure

• Monomeric actin subunits• ATP bound to actin subunits• Two helical protofilaments• Actin subunits in same orientation

Actin Filament Structure

Plus And Minus Ends Of Actin Filaments And Microtubules

• Polarity from regular orientation of subunits• During elongation, subunits added preferentially to plus end

Nucleotide Hydrolysis

• Free actin and tubulin subunits are triphosphate form• Association with filament stimulates nucleotide hydrolysis• Diphosphate form more likely to dissociate from end• ATP/GTP caps dependent upon rate of addition

Treadmilling

• Subunits assembled at plus end and disassembled at minus end at same rate; triphosphate cap at plus end

• Subunits travel through filament

Dynamic Instability

• Alternating states of elongation and depolymerization• Dependent on presence or hydrolysis of triphosphate cap

Drugs Affecting Actin And Microtubules

• Toxins from plants specifically affect polymerization or depolymerization

• Taxol stabilizes microtubules, used in cancer therapy

Intermediate Filament Assembly

• Dimer has central regions wound into coiled-coil

• Staggered side-by-side arrangement of two dimers forms tetramer

• Tetramer is basic subunit for assembly of filaments

Types Of Intermediate Filaments

Type Component Polypeptides Cellular Location

Epithelial type I keratins (acidic) epithelial cells and type II keratins (basic) their derivatives

Axonal neurofilament proteins neurons

Nuclear lamins A, B, and C inside surface of nuclear membrane

Vimentin-like vimentin and related variousproteins

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Keratin Genetic Diseases

Epidermolysis bullosa simplex• Defective keratin in basal layer of epidermis• Rupture of cells and blistering from mechanical stress

Polymerization Of Actin And Microtubules In Vitro

• Nucleation: slow formation of stable oligomers• Elongation: rapid addition to filament• Steady-state: same rates of polymerization and depolymerization

• Centrosome: site of nucleation

– Multiple -tubulin ring complexes

– Pair of centrioles

• Nucleate from -tubulin ring complex

Nucleation Of Microtubules

Nucleation Of Actin Filaments

• Nucleate from actin-related protein complex

Regulating Filament Elongation

• Thymosin: sequesters actin monomers• Profilin: competes with thymosin, promotes actin assembly• Other proteins bind free tubulin

• Organization into bundles or gel-like networks

• Actin-binding bundling and gel-forming proteins

Cross-Linking Actin Filaments

Myosin: Actin Based Motor Proteins

• Head domain uses ATP hydrolysis to move toward plus end• Myosin II: two heavy chains each with head domain• Myosin I: one head domain

Microtubule Based Motor Proteins

• Kinesin: moves toward plus end• Dynein: moves toward minus end;

cytoplasmic and axonemal (ciliary) types

• Cycle of ATP binding, ATP hydrolysis, and phosphate release

Mechanism Of Myosin Movement

• 9 + 2 arrangement of microtubules; outer doublets• Cross-linking proteins• Ciliary dynein

Microtubules In Cilia And Flagella

• Motor force of dynein converted to bending motion

Movement Of Cilia And Flagella

Basal Bodies

• Organizing center for microtubules in cilia and flagella• Nine triplet microtubules

• Polymerization of actin causes protrusion at front

• New contacts form with solid surface

• Back of cell contracts

Cell Migration