Cytoskeletal Structural ProteinsPolypeptide types:

Actin a microfilaments (70 Å diameter),Keratin an intermediate filament (100 Å diameter)Tubulin a microtubule (240 Å diameter)

Actin Microfilament Assembly

F-actin (filamentous) and G-actin (globular)

Polarity with ATP-binding end the negative (-) end

Greater addition on the positive (+) end

Single subunit

Microfilament Treadmilling

Crawling Cells via Microfilament Treadmilling

Upturned Leading edge

Highly Dense Highly Dense Leading edgeLeading edge

Cytoskeletal Structural ProteinsPolypeptide types:

Actin a microfilaments (70 Å diameter),Keratin an intermediate filament (100 Å diameter)Tubulin a microtubule (240 Å diameter)

Intermediate Filaments

Basic structural unit: dimer of α-helices that wind around each other – coiled coil

Includes soft keratins that define internal structures and hard keratins of skin, hair and claws

Exclusively structural proteins

Dead epidermal skin Dead epidermal skin cells mostly keratincells mostly keratin

Residue Arrangement in a Coiled Coil StructureSeven amino acid pseudo repeat with positions 1 and 4 hydrophobic residues

Hydrophobic strip along the side of the helix

Intermediate Filament Dimer Model

Human hair

Special Intermediate Filament for Large Animals: Collagen

Triple helix extracellular matrix

Holds cells, organs and bones together

Rich in Gly (30%), Pro (30%) and hydroxyproline (30%)

Collagen Triple Helix Cross Section

A narrow left-handed single helix but

A right-handed triple helix

Collagen Single and Triple Helix Structure

Pro drives the left handed configuration

Only Gly can fit in the center position

Reverse winding strengthens the helix

Lys Side Chain Cross Linking

Collagen stronger than steel on a per weight basis

Cytoskeletal Structural ProteinsPolypeptide types:

Actin a microfilaments (70 Å diameter),Keratin an intermediate filament (100 Å diameter)Tubulin a microtubule (240 Å diameter)

Tubulin Forms Hollow Microtubules

Tubular structure provides greater strength

β-Tubulin and Tubulin Dimer

Describe the secondary structures of beta-tubulin.

Guanosine nucleotide-sites with α-subunit site solvent inaccessible

Microtubules Function in Cell Division

Draws chromosomes apart before cell division

Polar structurethat grows morerapidly at the (+)end

Microtubule treadmilling can occur

Drug-Mediated Microtubule DepolymerizationColchicine binds between the alpha and beta subunits

Drug-Mediated Microtubule Polymerization

Taxol stabilizes beta-tubulin subunits preventing depolymerization

Blocks rapidly dividing cells such as tumor cells

Motor Protein

Thick Filament

Actin interacts with the thick filament of myosin causing muscle contraction

Myosin Head and Neck Region

Binding of ATP to Myosin Releases Myosin from Actin

ATP Hydrolysis

Causes a myosin conformational shift and

Increases myosin binding to actin

Myosin binds to actin farther alongthe thin filament

Myosin binding to actin causes a release of phosphate and ADP

Power Stroke

Chapter 5 Problems:

1, 3, 5, 7, 9, 11, 13, and 17, 20, 25, 51