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

Area of Science:

• Cellular Biology
• Biochemistry
• Biophysics

Background:

• Actin filament assembly and organization are crucial for cellular processes like locomotion and division.
• WASP/Scar family proteins, activated by Rho family G proteins, stimulate the Arp2/3 complex to initiate actin nucleation.

Purpose of the Study:

• To investigate the molecular mechanisms of Arp2/3-dependent actin nucleation.
• To understand how different signaling pathways activating Arp2/3 generate distinct actin network architectures.

Main Methods:

• Direct comparison of Arp2/3 complex activity with WASP, N-WASP, and Scar1 activators.
• Kinetic analysis of actin assembly using purified proteins and cell extracts.
• Mathematical modeling of nucleation kinetics.
• Chemical crosslinking assays to study protein interactions and conformational changes.

Main Results:

• WASP, N-WASP, and Scar1 exhibit unique actin nucleation kinetics, with N-WASP showing the highest activity and Scar1 the lowest.
• Differences in nucleation rates are attributed to variations in rate constants for the activation step, influenced by C-terminal acidic domains.
• N-WASP and Scar1 induce conformational changes in Arp2/3 but with different crosslinking efficiencies to subunits p18 and p14.

Conclusions:

• The WA domains of WASP family proteins bind actin and Arp2/3 with similar affinities but yield a ~100-fold range in nucleation rates.
• Specific C-terminal acidic amino acid content tunes the actin filament formation rate for each protein.
• Arp2/3 complex regulation is not a simple on/off switch; precise rate tuning by nucleation-promoting factors dictates actin network architecture.