A polar, filamentous polymer of globular actin subunits (F-actin) that assembles into helical two-stranded filaments and contributes to cell shape, mechanical resistance, force generation, and motility in muscle and nonmuscle cells.

Principle

ATP-bound G-actin monomers polymerize head-to-tail into a polar F-actin filament with chemically and kinetically distinct plus (barbed) and minus (pointed) ends; dynamics (nucleation, elongation, severing, and annealing) regulated by actin-binding proteins and ATP hydrolysis produce treadmilling and force for protrusion and contraction.

Demonstration

In a migrating fibroblast, a branched network of actin filaments (Arp2/3-nucleated) pushes the plasma membrane forward at the lamellipodium while bundled, myosin II-containing stress fibers generate contractile tension; in skeletal muscle, ordered arrays of F-actin form thin filaments that slide past myosin thick filaments during contraction.

Misapplication

Describing microtubules or intermediate filaments as actin filaments, or attributing contractile force solely to F-actin without acknowledging the requirement for myosin motor activity and crosslinkers.

Consequence

Proper F-actin dynamics enable directed cell migration, cytokinesis, maintenance of cortical tension, endocytosis, and mechanotransduction; dysregulation alters cell polarity, impairs movement, and can change tissue mechanics.

Reversal

Net depolymerization or blockade of actin polymerization converts a dynamic, load-bearing cytoskeleton into a softer, less polarized state; conversely, stable, hyperbundled F-actin networks resist remodeling but may limit adaptability.

Boundary

Refers specifically to polymerized actin (F-actin); excludes monomeric G-actin, non-actin cytoskeletal polymers (microtubules, intermediate filaments), and extracellular fibrous proteins; bacterial actin homologs perform related roles but differ in sequence and regulation.

Semantic Tension

‘Actin filament’ emphasizes chemical identity and filamentous state, while the synonym ‘microfilament’ stresses size and cytoskeletal role; ‘F-actin’ highlights polymerization status—terms overlap but carry different experimental connotations.

Synthesis

Actin filaments are polarized, ATP-dependent polymers of actin whose regulated assembly and interaction with binding proteins and myosin produce force, shape, and dynamic remodeling of cells across diverse contexts.