Functional Modules

    Nucleation Promoting Factors and NPF Accessory Proteins regulate actin filament polymerization[Edit]

    Additional proteins are required for the initiation and regulation of actin filament polymerization. These include the Nucleation Promoting Factors (NPF) and NPF Accessory Proteins.

    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 free actin (which inhibits actin nucleotide exchange) [1,2, 3, 4]; these combined functions promote actin-filament assembly at the barbed end. 
    Mammalian NPFs are broadly grouped into 2 classes: 

    Figure 1. WASP_WAVE_nucleation_promoting_factors: The molecular organization of Scar/WAVE family members and WASP, with examples of how the NPFs are represented in figures throughout this resource. Relevant domains believed to be important for binding to actin and for protein-protein interactions are highlighted (reviewed in [5, 6]). WHD= WAVE (also known as SCAR)-homology domain; WIP= WASP-interacting protein; V= verprolin-homology domain (i.e. WASP-homology-2 domain [WH2]); C= cofilin-homology domain; A= acidic-rich domain; WH1= WASP-homology-1 (aka Ena-VASP-homology-1 [EVH1]); GBD= GTPase binding domain. Of note: N-WASP contains an additional V domain, and the Scar/WAVE proteins lack a direct binding domain for the Rho GTPase family.
    Class I proteins are classified into five groups:
    1) Wiskott-Aldrich Syndrome protein (WASP) and neuronal-enriched homologue of WASP (N-WASP)
    2) WASP family Verprolin-homologous (WAVE) proteins (aka Suppressor of cAMP receptor [Scar])
    3) WASP and Scar homologue (WASH)
    4) WASP homologue associated with actin, membranes and microtubules (WHAMM)
    5) junction-mediating regulatory protein (JMY)

    Class II includes proteins such as cortactin.

    The conserved verpolin-cofilin-homology and acidic-rich (VCA) domain at the carboxy terminus of WASP and Scar family members binds directly to the Arp2/3 complex to increase its nucleation activity [7, 8, 9, 10]. WASp also associates with other signaling components (e.g. haemopoietic cell kinase) and formins to modulate actin polymerization for cell polarization and chemotaxis in neutrophils [11, 12]. WASp and formins also cooperate to control the balance between lamellipodial protrusion activity in epithelial cells [13]. The WASP family of NPFs are normally auto-inhibited due to protein interactions which prevent the NPF from associating with actin and the Arp2/3 complex; they are activated by Rho GTPases and PIP2 [8] (reviewed in [14, 15]). In contrast, the Scar/WAVE proteins have constitutive activity [16]. NPF accessory proteins also modulate the activity of NPFs.

    NPF Accessory Proteins

    Accessory proteins bind to WASp and Scar/WAVE proteins to modulate their NPF activity.

    Examples of NPF accessory proteins include Verprolin (yeast), which modulates the activity of WASp with type I myosins, to promote actin assembly by Arp2/3 complex [17]. WASp-interacting proteins (WIPs) will also regulate the WASp activity. For example WIP [18] not only inhibits N-WASP, but also promotes nucleation and activation of the Arp2/3 complex through the coordinated binding of actin and another NPF, cortactin [19]. SPIN90/WISH (SH3 protein interacting with Nck, 90 kDa/WASP-interacting SH3 protein) [20]) on the other hand increases actin assembly in dendritic filopodia/spines independently of N-WASP through its association with the neuron-specific scaffolding protein, PSD-95 [20]. Surprisingly, indirect evidence shows that WIPs are required for WASp function [21].

    In another example, the WAVE complex will inhibit Scar/WAVE proteins from activating the Arp2/3 complex. The NPF activity of Scar/WAVE is restored during nucleation when Rac-GTP causes the dissociation of the WAVE complex from WAVE1 [22]. Similarly, IRSp53 has been implicated in both lamellipodia and filopodia formation/protrusion by augmenting the Rac-GTP-induced activation of WAVE NPF activity [23, 24]. 





    References

    1. Chereau D., Kerff F., Graceffa P., Grabarek Z., Langsetmo K., Dominguez R. Actin-bound structures of Wiskott-Aldrich syndrome protein (WASP)-homology domain 2 and the implications for filament assembly. Proc. Natl. Acad. Sci. U.S.A. 2005; 102(46). [PMID: 16275905]
    2. Suetsugu S., Miki H., Takenawa T. The essential role of profilin in the assembly of actin for microspike formation. EMBO J. 1998; 17(22). [PMID: 9822597]
    3. Higgs HN., Blanchoin L., Pollard TD. Influence of the C terminus of Wiskott-Aldrich syndrome protein (WASp) and the Arp2/3 complex on actin polymerization. Biochemistry 1999; 38(46). [PMID: 10563804]
    4. Egile C., Loisel TP., Laurent V., Li R., Pantaloni D., Sansonetti PJ., Carlier MF. Activation of the CDC42 effector N-WASP by the Shigella flexneri IcsA protein promotes actin nucleation by Arp2/3 complex and bacterial actin-based motility. J. Cell Biol. 1999; 146(6). [PMID: 10491394]
    5. Bose KS., Sarma RH. Delineation of the intimate details of the backbone conformation of pyridine nucleotide coenzymes in aqueous solution. Biochem. Biophys. Res. Commun. 1975; 66(4). [PMID: 2]
    6. Järvisalo J., Saris NE. Action of propranolol on mitochondrial functions–effects on energized ion fluxes in the presence of valinomycin. Biochem. Pharmacol. 1975; 24(18). [PMID: 13]
    7. Machesky LM., Mullins RD., Higgs HN., Kaiser DA., Blanchoin L., May RC., Hall ME., Pollard TD. Scar, a WASp-related protein, activates nucleation of actin filaments by the Arp2/3 complex. Proc. Natl. Acad. Sci. U.S.A. 1999; 96(7). [PMID: 10097107]
    8. Rohatgi R., Ma L., Miki H., Lopez M., Kirchhausen T., Takenawa T., Kirschner MW. The interaction between N-WASP and the Arp2/3 complex links Cdc42-dependent signals to actin assembly. Cell 1999; 97(2). [PMID: 10219243]
    9. Yarar D., To W., Abo A., Welch MD. The Wiskott-Aldrich syndrome protein directs actin-based motility by stimulating actin nucleation with the Arp2/3 complex. Curr. Biol. 1999; 9(10). [PMID: 10339430]
    10. Miki H., Yamaguchi H., Suetsugu S., Takenawa T. IRSp53 is an essential intermediate between Rac and WAVE in the regulation of membrane ruffling. Nature 2000; 408(6813). [PMID: 11130076]
    11. Shi Y., Dong B., Miliotis H., Liu J., Alberts AS., Zhang J., Siminovitch KA. Src kinase Hck association with the WASp and mDia1 cytoskeletal regulators promotes chemoattractant-induced Hck membrane targeting and activation in neutrophils. Biochem. Cell Biol. 2009; 87(1). [PMID: 19234535]
    12. Shi Y., Zhang J., Mullin M., Dong B., Alberts AS., Siminovitch KA. The mDial formin is required for neutrophil polarization, migration, and activation of the LARG/RhoA/ROCK signaling axis during chemotaxis. J. Immunol. 2009; 182(6). [PMID: 19265163]
    13. Sarmiento C., Wang W., Dovas A., Yamaguchi H., Sidani M., El-Sibai M., Desmarais V., Holman HA., Kitchen S., Backer JM., Alberts A., Condeelis J. WASP family members and formin proteins coordinate regulation of cell protrusions in carcinoma cells. J. Cell Biol. 2008; 180(6). [PMID: 18362183]
    14. Le Clainche C., Carlier MF. Regulation of actin assembly associated with protrusion and adhesion in cell migration. Physiol. Rev. 2008; 88(2). [PMID: 18391171]
    15. Campellone KG., Welch MD. A nucleator arms race: cellular control of actin assembly. Nat. Rev. Mol. Cell Biol. 2010; 11(4). [PMID: 20237478]
    16. Machesky LM., Insall RH. Scar1 and the related Wiskott-Aldrich syndrome protein, WASP, regulate the actin cytoskeleton through the Arp2/3 complex. Curr. Biol. undefined; 8(25). [PMID: 9889097]
    17. Sirotkin V., Beltzner CC., Marchand JB., Pollard TD. Interactions of WASp, myosin-I, and verprolin with Arp2/3 complex during actin patch assembly in fission yeast. J. Cell Biol. 2005; 170(4). [PMID: 16087707]
    18. Ramesh N., Antón IM., Hartwig JH., Geha RS. WIP, a protein associated with wiskott-aldrich syndrome protein, induces actin polymerization and redistribution in lymphoid cells. Proc. Natl. Acad. Sci. U.S.A. 1997; 94(26). [PMID: 9405671]
    19. Kinley AW., Weed SA., Weaver AM., Karginov AV., Bissonette E., Cooper JA., Parsons JT. Cortactin interacts with WIP in regulating Arp2/3 activation and membrane protrusion. Curr. Biol. 2003; 13(5). [PMID: 12620186]
    20. Lee S., Lee K., Hwang S., Kim SH., Song WK., Park ZY., Chang S. SPIN90/WISH interacts with PSD-95 and regulates dendritic spinogenesis via an N-WASP-independent mechanism. EMBO J. 2006; 25(20). [PMID: 16990791]
    21. Rajmohan R., Meng L., Yu S., Thanabalu T. WASP suppresses the growth defect of Saccharomyces cerevisiae las17Delta strain in the presence of WIP. Biochem. Biophys. Res. Commun. 2006; 342(2). [PMID: 16488394]
    22. Eden S., Rohatgi R., Podtelejnikov AV., Mann M., Kirschner MW. Mechanism of regulation of WAVE1-induced actin nucleation by Rac1 and Nck. Nature 2002; 418(6899). [PMID: 12181570]
    23. Suetsugu S., Kurisu S., Oikawa T., Yamazaki D., Oda A., Takenawa T. Optimization of WAVE2 complex-induced actin polymerization by membrane-bound IRSp53, PIP(3), and Rac. J. Cell Biol. 2006; 173(4). [PMID: 16702231]
    24. Mattila PK., Pykäläinen A., Saarikangas J., Paavilainen VO., Vihinen H., Jokitalo E., Lappalainen P. Missing-in-metastasis and IRSp53 deform PI(4,5)P2-rich membranes by an inverse BAR domain-like mechanism. J. Cell Biol. 2007; 176(7). [PMID: 17371834]
    Updated on: Wed, 26 Feb 2014 10:54:15 GMT
    History