FAQs

Store both G-actin and F-actin solutions on ice (0 °C). Avoid storage in a refrigerator (> +4 °C) or freezer (< −20 °C) for working solutions.

G-actin: typically stable for ~1 week on ice. Older solutions may remain polymerizable, but can accumulate nuclei/oligomers that change polymerization kinetics—therefore not recommended for kinetic measurements. If sterile (bacteria-free), they can still be suitable for certain endpoint F-actin assays (e.g., spin-down).

F-actin: generally more stable. Filaments can be stored on ice for 2 weeks or longer if bacterial growth is prevented. For longer storage, refresh ATP periodically; a practical approach is adding 0.2 mM ATP (pH 7.4) weekly.
 

Use ultrapure, 0.2 µm-filtered water for reconstitution and minimize bacterial introduction—actin buffers typically contain ATP, which supports bacterial growth.

When supplied freeze-dried, the actin solution is sterile-filtered prior to lyophilization (0.2 µm). Maintain sterility after opening, keep solutions on ice, and avoid repeated warming cycles.
 

G-actin (monomeric actin) polymerizes under physiological buffer conditions to form F-actin. Polymerization is coupled to ATP hydrolysis and produces a double-helical filament with intrinsic polarity.

In vitro, filaments grow from both ends at different rates: the fast-growing plus end and the slow-growing minus end. Polymerization depends on monomer concentration and solvent conditions.

Below the critical concentration (Cc), actin does not polymerize. Above Cc, polymerization proceeds until free monomer approaches the Cc defined by filament ends. Note: actin isoforms can have different Cc values.

If you need a monomeric G-actin starting state (e.g., for kinetics), remove nuclei/oligomers by either ultracentrifugation or gel filtration.

F-actin can be pelleted by ultracentrifugation at ~100,000×g for 3 hours at approximately 15 °C. At lower temperature, Cc increases, which can raise free monomer levels; moderate temperature can improve filament yield in some workflows.
 

Actin treadmilling is a steady state where a filament adds subunits at the plus end while losing subunits at the minus end, driven by different critical concentrations at the two ends and ATP/ADP-state changes in filament subunits.

Mechanistically, treadmilling arises because plus and minus ends have different association/dissociation kinetics. ATP-actin incorporation is followed by ATP hydrolysis and phosphate release within the filament, which shifts stability and favors dissociation of older subunits (often enriched toward the minus end).

Why it matters: Even if a filament looks “stable” in bulk, treadmilling can create continuous subunit flux. This affects quantitative interpretation in kinetics assays, filament-length distributions, and comparisons across buffer conditions and actin isoforms.
 

Actin assembly is highly buffer-dependent: changes in ionic strength and divalent cations (Mg²⁺/Ca²⁺) shift polymerization kinetics and the critical concentration, which alters filament number and length distributions.

In vitro, the balance between monomer association and dissociation at filament ends depends on electrostatics (salt), specific cation interactions, and the actin nucleotide state. As a result, the same nominal actin concentration can produce very different filament populations when salts/cations differ.

Actin turnover is driven by regulated disassembly pathways; the ADF/cofilin family is a key factor that promotes actin network turnover, supporting rapid remodeling of the cytoskeleton.

In cells, filament disassembly is controlled by multiple actin-binding proteins and depends on network architecture, nucleotide state, and associated factors. ADF/cofilin proteins are widely described as central components of the machinery that accelerates actin network turnover to sustain dynamics.

Experimental note: When interpreting perturbations (e.g., drugs or ABPs), distinguish effects on monomer availability, nucleation, elongation, severing, and end-capping—different mechanisms can yield similar bulk readouts.