What factors regulate actin filament polymerization? Nucleation Promoting Factors (NPFs) (e.g. WASP, Scar/WAVE) modulate actin filament nucleation by bringing together actin monomers and pre-existing actin filaments, for example, during filopodial initiation where they recruit the Arp2/3 complex. NPFs compete with profilin for binding to [...]

Actin Crosslinking Crosslinking of actin filaments is a critical step in cell motility and is a fundamental process in filopodia protrusion and lamellipodia formation. Smaller cross-linking proteins that are more globular (e.g. fascin) or have more than one actin binding site (e.g. fimbrin, [...]

Fimbrin Fimbrin (aka plastin homologue, accumentin) is an actin binding protein that was originally identified in microvilli [1][2]. This schematic diagram illustrates the molecular organization of fimbrin as depicted in this resource, and highlights the relevant domains for binding to actin filaments. [...]

I-BAR and Other Proteins/Factors Proteins containing I-BAR (inverted Bin/amphiphysin/Rvs i.e. IRSp53 Missing-in-metastasis homology Domain or IMD) cooperate with various components of actin filament assembly, to promote filopodia protrusion, via several mechanisms including the stimulation of F-actin crosslinking [1]. This schematic diagram illustrates the [...]

What is capping protein? Capping protein is involved in actin filament assembly and disassembly Capping proteins control access to the free barbed ends of actin filaments and is therefore a major factor affecting actin filament elongation. Capping proteins have a high affinity for barbed [...]

What are intermediate filaments? Intermediate filaments are a primary component of the cytoskeleton, although they are not found in all eukaryotes, and are absent in fungi and plants [1]. These filaments, which extend throughout the cytoplasm and inner nuclear membrane are composed from a [...]

How are intermediate filaments assembled? The soluble subunit for creating intermediate filaments is a tetramer. The tetramer is created from monomers in a stepwise fashion (as reviewed in [1]). First, two monomers associate via their central domains to form parallel helical coils around each [...]

What are microtubules? Microtubules are hollow cylinders [1] that are approximately 25nm in diameter [2] and vary in length from 200 nm to 25 μm. They are formed by the lateral association of between 12 and 17 protofilaments into a regular helical lattice [2],[3]. [...]

Alpha-actinin α-actinin is an actin-binding protein [1] and component of the actin crosslinking functional modules; it lacks G-actin binding activity and lacks actin initiation/nucleation activity [2]. α-actinin is an important organizer of the cytoskeleton that belongs to the spectrin superfamily (which includes spectrin, dystrophin, [...]

Fascin Fascin is the major actin crosslinking protein found in a wide range of filopodia [1][2][3]. This protein has been shown to work in concert with other cross linkers such as α-actinin to produce filopodia, although fascin itself is sufficient to form filopodia-like bundles [...]

Filamin The filamin family of proteins bind to both actin and a number of signaling molecules including Rho GTPases. Evidence for this was shown with the loss of Filamin-A in M2 Melanoma cells, which prevented RalA- and Cdc42- mediated filopodia formation [1]. Filamin A [...]

What are Contractile Fibers? Certain myosin isoforms (i.e. myosin II) form bipolar assemblies via the extended coiled-coil domains in the heavy chains (see also “thick filaments”). Actin “thin filaments” with opposite polarity associate with thick filaments to create contractile bundles that can be found [...]