Unit 06 Pt2

Actin Structure
- Composed of actin subunits (monomers) called G actin (globular actin).
- Each G actin is a polypeptide chain of ~375 amino acids.
- Binds to ATP and is conserved among eukaryotic cells.
- Different genes or isoforms for tissue specificity exist but aren't covered in detail.
Filament Formation
- G actin monomers polymerize to form filamentous actin (F actin).
- F actin has head-to-tail orientation and typically forms a right-handed helix, diameter < 10 nm (5-9 nm).
- The assembly is polarized with a slow-growing minus end and a fast-growing plus end.
- ATP binding pocket is oriented towards the minus end.

Critical Concept: Nucleation
- Essential for rapid polymerization; oligomers serve as nucleation points.
- A critical minimum of monomers is required for growth.
- Large oligomers facilitate faster filament growth compared to individual subunits which may disassemble easily.
Critical Concentration
- Defined as the point where monomeric subunits are in equilibrium with the polymer.
- Growth occurs at the plus end while loss occurs at the minus end, influenced by differential kinetics and conformational changes.

On/Off Rates and ATP Hydrolysis
- Rates of addition (polymerization) are different at the plus and minus ends due to ATP hydrolysis.
- G actin exists in ATP-bound (T form) or ADP-bound (D form) states.
- D form has a lower affinity for neighbors, leading to favoring of dissociation.
- critical concentrations differ between T (lower) and D forms (higher).
Treadmilling
- Characterizes simultaneous growth at the plus end and shrinkage at the minus end.
- This dynamic process keeps the filament length stable while allowing for rapid cycling of subunits.

Accessory and Regulatory Proteins
- Small proteins regulate filament length, geometry, and stability.
- G-actin's half-life: 30 min (in vitro), 30 sec (in vivo) due to stabilizing accessory proteins.
- Examples:
  - Thymosin: Binds G actin to prevent polymerization.
  - Profilin: Promotes polymerization by stabilizing ATP-bound G actin.
  - ARP 2/3 complex: Important for nucleation and generating branched networks by binding to the minus end.
  - Formins: Facilitate linear filament growth at the plus end.

Stabilizing Proteins
- Tropomyosin: Stabilizes actin filaments, critical for contraction.
- Capping Proteins: Store polymerization, e.g., Tropomodulin caps the minus end.
Severing Proteins
- Examples:
  - Gelsolin: Severing actin filaments under high Ca2+ concentrations.
  - Cofilin: Induces torsion to dismantle older filaments (D form).

Types of Actin Structures
- Dendritic Networks: Formed by ARP 2/3 promoting branching and gel-like structures.
- Cable-like Bundles: Formed by formins resulting in direct, unbranched arrangements.
Modes of Cell Migration
- Mesenchymal Mode: Involves focal adhesions and traction through lamellipodia.
- Amoeboid Mode: Rapid movement via explosive actin polymerization at leading edge.
- Blebbing: Protrusions formed by hydrostatic pressure from plasma membrane detachment.

Non-Contractile vs. Contractile Bundles
- Fimbrin: Creates tightly packed bundles, preventing myosin interaction.
- Alpha-Actinin: Forms looser arrangements allowing myosin to enter and facilitate contraction.
- Gel-forming Proteins: Clamps actin at right angles for viscosity in structures such as lammelopodia.

Anchor to Plasma Membrane
- Spectrin: Forms a web beneath the plasma membrane, providing cell shape and flexibility.
- Membrane Proteins: Regulate transduction pathways and maintain cellular organization.