Abi1 is essential for the formation and activation of a WAVE2 signalling complex

Abstract

WAVE2 belongs to a family of proteins that mediates actin reorganization by relaying signals from Rac to the Arp2/3 complex, resulting in lamellipodia protrusion. WAVE2 displays Arp2/3-dependent actin nucleation activity in vitro, and does not bind directly to Rac. Instead, it forms macromolecular complexes that have been reported to exert both positive and negative modes of regulation. How these complexes are assembled, localized and activated in vivo remains to be established. Here we use tandem mass spectrometry to identify an Abi1-based complex containing WAVE2, Nap1 (Nck-associated protein) and PIR121. Abi1 interacts directly with the WHD domain of WAVE2, increases WAVE2 actin polymerization activity and mediates the assembly of a WAVE2-Abi1-Nap1-PIR121 complex. The WAVE2-Abi1-Nap1-PIR121 complex is as active as the WAVE2-Abi1 sub-complex in stimulating Arp2/3, and after Rac activation it is re-localized to the leading edge of ruffles in vivo. Consistently, inhibition of Abi1 by RNA interference (RNAi) abrogates Rac-dependent lamellipodia protrusion. Thus, Abi1 orchestrates the proper assembly of the WAVE2 complex and mediates its activation at the leading edge in vivo.

Activation of Arp2/3 complex-dependent actin polymerization by plant proteins distantly related to Scar/WAVE

“…At their N termini, AtSCAR1 to AtSCAR4, along with a fifth protein (At4g18600), share 26-27% identity with the Nterminal SHDs of animal Scar͞WAVE proteins. The function of the SHD of Scar͞WAVE is not well understood, but the SHD of WAVE2 has recently been shown to bind directly to Abi-1, a core component of the WAVE regulatory complex that is critical for complex assembly (11,25). A family of four predicted Arabidopsis genes encoding proteins with a low level of homology to mammalian Abi-1 has recently been identified (16); the products of these genes may bind to the SHDs of AtSCAR proteins and play a role in assembly of an AtSCAR regulatory complex.…”

EMT, the cytoskeleton, and cancer cell invasion

“…Here, actin assembly and lateral branching (dendritic nucleation) are mainly controlled by the Arp2/3 complex, a seven subunit protein and major initiator of actin assembly. The activity of the Arp2/3 complex itself is controlled by actin nucleation-promoting factors, such as the N-WASP or the Scar/WAVE complexes which are themselves recruited to and activated at the membrane by Rac1 [172][173][174]. Increased expression of Arp2/3 and WAVE2 has been shown to correlate with poor prognosis in breast and liver carcinomas underlining the relevance of lamellipodia-like structures in cancer progression [175,176].…”

The interplay between Src and integrins in normal and tumor biology

“…Rac and Cdc42 activation leads to the activation of the WAVE and WASP family of proteins, in turn promoting activation of the Arp2/3 complex. Activation of the Arp2/3 complex serves to initiate new actin filament formation, forming a branched column, which pushes the leading edge forward (Higgs and Pollard, 2000;Eden et al, 2002;Innocenti et al, 2004). This is likely a major mechanism by which Src regulates motility in response to integrin signals.…”