Actin filament

A actin filament assembled from actin subunits. actin filament mass and actin filament number are distinct quantities; subunit exchange can continue while the total mass is approximately steady.

An actin-binding molecule used in fluorescent conjugates to visualize filamentous actin. Weak staining must be distinguished from background, and staining accessibility can affect inference about residual polymer.

Binding that restricts subunit exchange at a actin filament end. actin filament capping and actin monomer sequestration can both reduce bulk assembly but have different microscopic mechanisms.

Breakage of a actin filament into shorter pieces. This changes actin filament number and end number and should be distinguished from loss of subunits at pre-existing ends.

A bundle of actin filaments that can support directed intracellular transport. Absence of a visible cable does not exclude sparse short actin filaments below an imaging threshold.

A localized actin assembly in budding yeast, commonly associated with endocytosis. The lifetime of a visible patch and the lifetime of an individual constituent actin filament are different measurements.

Replacement and redistribution of subunits through assembly and disassembly. A persistent structure can be highly dynamic even when its appearance is stable.

Loss of subunits from actin filaments. Blocking replacement from soluble actin monomers can cause net actin filament loss without direct drug binding to the actin filament.

Assembly of actin subunits into actin filaments. An unseeded bulk reaction can show a actin nucleation lag, a growth phase and an approximately constant final polymer mass; equal endpoints need not imply equal kinetics.

A fluorescence readout of actin assembly using polymerization-dependent signal from a labelled actin fraction. Label effects, direct fluorescence changes, quenching and background must be controlled.^[ref]

The free assembly-competent actin monomer concentration at which net addition at a specified actin filament end is zero in a specified model: $C_c=k_{-}/k_{+}$. Different ends and nucleotide conditions can have different values.

Net addition of subunits at a actin filament end. A simple end model gives $v=k_{+}[G]-k_{-}$, with the sign changing at the relevant actin critical concentration.

Formation of a stable assembly from which an actin filament can grow. Lowering the concentration of assembly-competent actin monomer can affect actin nucleation much more strongly than addition to an existing end.