01 — Actin: Structure, Polymerization, and Dynamics

Prerequisite Concepts

• Protein structure (primary through quaternary)
• Nucleotide binding and hydrolysis (ATP → ADP + Pi)
• Basic thermodynamics: equilibrium, rate constants, free energy

Core Explanation

G-Actin and F-Actin

Actin exists in two forms: globular (G-actin) monomers and filamentous (F-actin) polymers. G-actin is a ~42 kDa protein with four subdomains arranged around a central cleft that binds ATP (or ADP). Under physiological salt conditions and above the critical concentration, G-actin polymerizes into F-actin — a helical polymer that can be described as two parallel protofilaments twisting around each other with a rise of ~2.7 nm per subunit and a helical repeat every ~37 nm (13 subunits per half-turn).

Polarity: Barbed and Pointed Ends

F-actin is inherently polar. All subunits within a filament orient in the same direction. This structural polarity creates two biochemically distinct ends:

• Barbed end (plus end): Faster-growing; the ATP-binding cleft faces outward. Subunits add and dissociate rapidly.
• Pointed end (minus end): Slower-growing; the cleft faces the filament interior. Subunit exchange is slower.

The "barbed" and "pointed" nomenclature comes from the arrowhead appearance when myosin S1 fragments decorate actin filaments in electron micrographs.

ATP Binding, Hydrolysis, and the Polymerization Cycle

Each G-actin monomer binds one molecule of ATP. The cycle proceeds as follows:

1. Nucleation (rate-limiting): Three G-actin monomers must assemble to form a stable seed (nucleus). This is thermodynamically unfavorable and is the key regulatory step in vivo.
2. Elongation: ATP-G-actin adds preferentially to the barbed end. Once incorporated, ATP is hydrolyzed to ADP-Pi, then Pi is slowly released. The resulting ADP-actin is less stable.
3. Steady state: At the critical concentration (Cc), the rates of subunit addition and loss are equal. For ATP-actin, Cc(barbed) ≈ 0.1 μM and Cc(pointed) ≈ 0.6 μM — a ~6-fold difference that drives treadmilling.

Treadmilling

When the free G-actin-ATP concentration falls between the Cc of the barbed end and the Cc of the pointed end, treadmilling occurs: subunits add at the barbed end while an equal number dissociate from the pointed end. The filament appears to "move" while maintaining constant length. This is not a perpetual motion machine — it consumes ATP hydrolysis energy and represents a steady-state flux, not equilibrium.

Key point: Treadmilling is a steady-state phenomenon requiring continuous ATP hydrolysis. It does not occur in pure ADP-actin or in the absence of nucleotide hydrolysis.

Critical Concentration (Cc)

The critical concentration is the free G-actin concentration at which the rates of assembly and disassembly are equal. Each end has its own Cc, and the difference between them drives net polymerization or depolymerization. In cells, the concentration of profilin-ATP-actin (~50–100 μM) far exceeds the Cc for barbed ends, ensuring rapid barbed-end growth when free ends are available.

Regulatory Proteins

Profilin: Nucleotide Exchange and Polymerization Promotion

Profilin binds G-actin at the barbed-end face and catalyzes the exchange of ADP for ATP (nucleotide exchange factor). Profilin–ATP-actin can add only to barbed ends — profilin physically blocks pointed-end addition. This channels actin monomers toward barbed-end growth and maintains the pool of polymerization-competent ATP-actin.

Cofilin: Severing and Turnover

Cofilin (ADF/cofilin family) binds preferentially to ADP-actin subunits within filaments, inducing a twist that severs the filament and increases the number of free ends. It also enhances the off-rate of ADP-actin from pointed ends. Cofilin activity accelerates filament turnover — essential for rapid remodeling during migration and endocytosis. Cofilin is inhibited by phosphorylation (LIM kinase pathway) and reactivated by dephosphorylation (slingshot phosphatases).

Arp2/3 Complex: Branched Networks

The Arp2/3 complex nucleates new filaments as branches off the sides of existing filaments (at a ~70° angle). Arp2 and Arp3 are actin-related proteins that mimic a dimeric actin nucleus. Activation requires nucleation-promoting factors (NPFs) such as WASP, N-WASP, and WAVE — themselves downstream of Rho-family GTPases (Cdc42, Rac). Arp2/3-generated branched networks power lamellipodial protrusion and pathogen motility (e.g., Listeria monocytogenes actin comet tails).

Formins: Unbranched Filaments

Formins nucleate and processively elongate unbranched (linear) actin filaments. They dimerize to form a donut-shaped ring that rides the growing barbed end, protecting it from capping proteins while allowing subunit addition. Formins are particularly important for filopodia, stress fibers, and the cytokinetic contractile ring. The formin homology 2 (FH2) domain provides the processive polymerase activity.

Capping Proteins

Barbed-end capping proteins (e.g., CapZ in muscle, heterodimeric capping protein) bind barbed ends and block both subunit addition and loss. Capping restricts growth to the subset of uncapped ends and increases the local G-actin concentration, indirectly promoting Arp2/3 nucleation. Capping "funnels" monomers toward the remaining free barbed ends, enabling rapid, directional growth of a subset of filaments.

Disease Connections

• Wiskott-Aldrich Syndrome: Mutations in WASP (an Arp2/3 activator) cause immunodeficiency and thrombocytopenia due to defective actin remodeling in hematopoietic cells.
• Cardiomyopathies: Mutations in cardiac actin and actin-binding proteins cause dilated and hypertrophic cardiomyopathies.
• Cancer metastasis: Dysregulation of Arp2/3, cofilin, and formins contributes to invasive cell migration.

Common Misconceptions and Exam Traps

• Wrong: "ATP hydrolysis drives actin polymerization." Correct: Polymerization is energetically favorable without hydrolysis; hydrolysis marks subunits for disassembly and enables treadmilling.
• Wrong: "Cofilin depolymerizes actin filaments." Correct: Cofilin severs filaments (creating more ends) and enhances subunit dissociation — it increases turnover, not simple depolymerization.
• Wrong: "Arp2/3 nucleates filaments de novo." Correct: Arp2/3 requires a pre-existing "mother" filament as a template for branch nucleation.
• Wrong: "Profilin promotes actin polymerization." Correct: It does — but only at barbed ends; profilin-ATP-actin cannot add to pointed ends.