Different WASP family proteins stimulate different Arp2/3 complex-dependent actin-nucleating activities

Curr Biol. 2001 Dec 11;11(24):1903-13. doi: 10.1016/s0960-9822(01)00603-0.

Abstract

Background: Assembly and organization of actin filaments are required for many cellular processes, including locomotion and division. In many cases, actin assembly is initiated when proteins of the WASP/Scar family respond to signals from Rho family G proteins and stimulate the actin-nucleating activity of the Arp2/3 complex. Two questions of fundamental importance raised in the study of actin dynamics concern the molecular mechanism of Arp2/3-dependent actin nucleation and how different signaling pathways that activate the same Arp2/3 complex produce actin networks with different three-dimensional architectures?

Results: We directly compared the activity of the Arp2/3 complex in the presence of saturating concentrations of the minimal Arp2/3-activating domains of WASP, N-WASP, and Scar1 and found that each induces unique kinetics of actin assembly. In cell extracts, N-WASP induces rapid actin polymerization, while Scar1 fails to induce detectable polymerization. Using purified proteins, Scar1 induces the slowest rate of nucleation. WASP activity is 16-fold higher, and N-WASP activity is 70-fold higher. The data for all activators fit a mathematical model in which one activated Arp2/3 complex, one actin monomer, and an actin filament combine into a preactivation complex which then undergoes a first-order activation step to become a nucleus. The differences between Scar and N-WASP activity are explained by differences in the rate constants for the activation step. Changing the number of actin binding sites on a WASP family protein, either by removing a WH2 domain from N-WASP or by adding WH2 domains to Scar1, has no significant effect on nucleation activity. The addition of a three amino acid insertion found in the C-terminal acidic domains of WASP and N-WASP, however, increases the activity of Scar1 by more than 20-fold. Using chemical crosslinking assays, we determined that both N-WASP and Scar1 induce a conformational change in the Arp2/3 complex but crosslink with different efficiencies to the small molecular weight subunits p18 and p14.

Conclusion: The WA domains of N-WASP, WASP, and Scar1 bind actin and Arp2/3 with nearly identical affinities but stimulate rates of actin nucleation that vary by almost 100-fold. The differences in nucleation rate are caused by differences in the number of acidic amino acids at the C terminus, so each protein is tuned to produce a different rate of actin filament formation. Arp2/3, therefore, is not regulated by a simple on-off switch. Precise tuning of the filament formation rate may help determine the architecture of actin networks produced by different nucleation-promoting factors.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Actin-Related Protein 2
  • Actin-Related Protein 3
  • Actins / chemistry
  • Actins / physiology*
  • Animals
  • Biopolymers
  • Cytoskeletal Proteins*
  • Humans
  • Microfilament Proteins / chemistry
  • Microfilament Proteins / physiology*
  • Nerve Tissue Proteins / chemistry
  • Nerve Tissue Proteins / physiology*
  • Protein Conformation
  • Proteins / chemistry
  • Proteins / physiology*
  • Wiskott-Aldrich Syndrome Protein
  • Wiskott-Aldrich Syndrome Protein Family
  • Wiskott-Aldrich Syndrome Protein, Neuronal

Substances

  • ACTR2 protein, human
  • ACTR3 protein, human
  • Actin-Related Protein 2
  • Actin-Related Protein 3
  • Actins
  • Biopolymers
  • Cytoskeletal Proteins
  • Microfilament Proteins
  • Nerve Tissue Proteins
  • Proteins
  • WAS protein, human
  • WASL protein, human
  • Wiskott-Aldrich Syndrome Protein
  • Wiskott-Aldrich Syndrome Protein Family
  • Wiskott-Aldrich Syndrome Protein, Neuronal