Filaments Of The Cytoskeleton Actin Filaments Helical polymers of protein actin Microtubules Hollow cylinders of protein tubulin Intermediate filaments Ropelike fibers of inter-mediate filament proteins

Microtubule Structure Tubulin subunits are heterodimers of a-tubulin and b-tubulin GTP bound to tubulin Hollow cylinder of 13 parallel protofilaments Tubulin subunits in same orientation

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

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 various proteins }

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

Nucleation Of Microtubules Nucleate from g-tubulin ring complex Centrosome: site of nucleation Multiple g-tubulin ring complexes Pair of centrioles

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

Cross-Linking Actin Filaments Organization into bundles or gel-like networks Actin-binding bundling and gel-forming proteins

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

Mechanism Of Myosin Movement Cycle of ATP binding, ATP hydrolysis, and phosphate release

Microtubules In Cilia And Flagella 9 + 2 arrangement of microtubules; outer doublets Cross-linking proteins Ciliary dynein

Movement Of Cilia And Flagella Motor force of dynein converted to bending motion

Basal Bodies Organizing center for microtubules in cilia and flagella Nine triplet microtubules

Cell Migration Polymerization of actin causes protrusion at front New contacts form with solid surface Back of cell contracts