Microfilament
[1] Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement.
[citation needed] In vitro actin polymerization, or nucleation, starts with the self-association of three G-actin monomers to form a trimer.
[citation needed] Subsequently, ADP-actin dissociates slowly from the pointed end, a process significantly accelerated by the actin-binding protein, cofilin.
End-capping proteins such as CapZ prevent the addition or loss of monomers at the filament end where actin turnover is unfavorable, such as in the muscle apparatus.
In most animal cells, monomeric actin is bound to profilin and thymosin beta-4, both of which preferentially bind with one-to-one stoichiometry to ATP-containing monomers.
[10] First found in neuronal axons, actin forms periodic rings that are stabilized by spectrin and adducin[11][12] – and this ring structure was then found by He et al 2016 to occur in almost every neuronal type and glial cells, across seemingly every animal taxon including Caenorhabditis elegans, Drosophila, Gallus gallus and Mus musculus.
Their formation and turnover are regulated by many proteins, including:[citation needed] The actin filament network in non-muscle cells is highly dynamic.
In the case of lamellipodial growth, the Arp2/3 complex generates a branched network, and in filopodia a parallel array of filaments is formed.
Various classes of myosin motors have very different behaviors, including exerting tension in the cell and transporting cargo vesicles.
[citation needed] One proposed model suggests the existence of actin filament barbed-end-tracking molecular motors termed "actoclampin".
[citation needed] The term actoclampin is derived from acto- to indicate the involvement of an actin filament, as in actomyosin, and clamp to indicate a clasping device used for strengthening flexible/moving objects and for securely fastening two or more components, followed by the suffix -in to indicate its protein origin.
[citation needed] Dickinson and Purich recognized that prompt ATP hydrolysis could explain the forces achieved during actin-based motility.
After binding to Glycyl-Prolyl-Prolyl-Prolyl-Prolyl-Prolyl-registers on tracker proteins, Profilin-ATP-actin is delivered ("loaded") to the unclamped end of the other sub-filament, whereupon ATP within the already clamped terminal subunit of the other subfragment is hydrolyzed ("fired"), providing the energy needed to release that arm of the end-tracker, which then can bind another Profilin-ATP-actin to begin a new monomer-addition round.
[15][17][18] The term actoclampin is generic and applies to all actin filament end-tracking molecular motors, irrespective of whether they are driven actively by an ATP-activated mechanism or passively.
[citation needed] Some actoclampins (e.g., those involving Ena/VASP proteins, WASP, and N-WASP) apparently require Arp2/3-mediated filament initiation to form the actin polymerization nucleus that is then "loaded" onto the end-tracker before processive motility can commence.
Then upon activation by ActA or VCA, the Arp complex is believed to undergo a major conformational change, bringing its two actin-related protein subunits near enough to each other to generate a new filament gate.
