Abl Interactor 1 (Abi-1) Wave-Binding and SNARE Domains Regulate Its Nucleocytoplasmic Shuttling, Lamellipodium Localization, and Wave-1 Levels

Abstract

The Abl interactor 1 (Abi-1) protein has been implicated in the regulation of actin dynamics and localizes to the tips of lamellipodia and filopodia. Here, we show that Abi-1 binds the actin nucleator protein Wave-1 through an amino-terminal Wave-binding (WAB) domain and that disruption of the Abi-1-Wave-1 interaction prevents Abi-1 from reaching the tip of the lamellipodium. Abi-1 binds to the Wave homology domain of Wave-1, a region that is required for translocation of Wave-1 to the lamellipodium. Mouse embryo fibroblasts that lack one allele of Abi-1 and are homozygous null for the related Abi-2 protein exhibit decreased Wave-1 protein levels. This phenotype is rescued by Abi-1 proteins that retain Wave-1 binding but not by Abi-1 mutants that cannot bind to Wave-1. Moreover, we uncovered an overlapping SNARE domain in the amino terminus of Abi-1 that interacts with Syntaxin-1, a SNARE family member. Further, we demonstrated that Abi-1 shuttles in and out of the nucleus in a leptomycin B (LMB)-dependent manner and that complete nuclear translocation of Abi-1 in the absence of LMB requires the combined inactivation of the SNARE, WAB, and SH3 domains of Abi-1. Thus, Abi-1 undergoes nucleocytoplasmic shuttling and functions at the leading edge to regulate Wave-1 localization and protein levels.

Essential role for Abi1 in embryonic survival and WAVE2 complex integrity

“…For example, in leukocytes, Abi1 was found in the WAVE2 complex and Abi2 in WAVE1 complex, whereas both Abi1 and Abi2 were present in WAVE2 complex in A431 cells (20). The basis for specificity of Abi protein incorporation into WAVE1 or WAVE2 complex is not known, although, based on high sequence conservation of binding sites, the affinities of the Abi proteins for WAVE proteins are expected to be similar (20,22,31). One explanation might be that Abi1 and Abi2 exhibit different patterns of threonine and serine phosphorylation (20), which is required for full activation of WAVE2 complex by Rac and acidic phospholipids (20).…”

CYFIP2, a Direct p53 Target, is Leptomycin-B Sensitive

“…Therefore, this data provides an important clue to the regulation and biological functions for both CYFIP2 and CYFIP1 that will need to be explored by future studies. It is of interest that many of the proteins with which CYFIP2 has been shown to interact or detected with in complex are known nucleocytoplasmic shuttling proteins including the FMR1, FXR1, and FXR2 RNA binding proteins from the FMR mRNP complex, 37 WAVE complex components like Abi-1 (and thus probably also the highly homologous protein, Abi-2), 38 as well as kinases that contribute to WAVE activation such as c-Abl. 39 The presence of an actin network within both the cytoplasm and nucleus is known 40,41 and WAVE complexes have been found previously within the nucleus.…”

c-Abl-mediated Phosphorylation of WAVE3 Is Required for Lamellipodia Formation and Cell Migration

“…The binding of CDC42, or SH3-containing proteins such as WISH and NCK to the proline-rich domain of N-WASP/WASP alleviates their inhibition by disrupting this intramolecular interaction, leading to the activation of N-WASP and WASP (8 -11). On the other hand, the activity of the WAVE proteins is believed to be regulated by their inclusion in a protein complex that also contains PIR121/NAP1/ Abi/HSPC300, which keeps them in an inactive state (12)(13)(14)(15)(16). This inhibition can be lifted by active small GTPases such as GTP-Rac (13), leading to the translocation of the complex to sites of active actin polymerization (14,16).…”