Abl is a nonreceptor tyrosine kinase and plays an essential role in the modeling and remodeling of F-actin by transducing extracellular signals. Abl and its paralog, Arg, are unique among the tyrosine kinase family in that they contain an unusual extended C-terminal half consisting of multiple functional domains. This structural characteristic may underlie the role of Abl as a mediator of upstream signals to downstream signaling machineries involved in actin dynamics. Indeed, a group of SH3-containing accessory proteins, or adaptor proteins, have been identified that bind to a proline-rich domain of the C-terminal portion of Abl and modulate its kinase activity, substrate recognition, and intracellular localization. Moreover, the existence of signaling cascade and biological outcomes unique to each adaptor protein has been demonstrated. In this paper, we summarize functional roles and mechanisms of adaptor proteins in Abl-regulated actin dynamics, mainly focusing on a family of adaptor proteins, Abi. The mechanism of Abl's activation and downstream signaling mediated by Abi is described in comparison with those by another adaptor protein, Crk.
The filamentous (F)-actin cytoskeleton is a fundamental component of all eukaryotic cells and plays an integral role in determining cell shape and locomotion. Thus, F-actin dynamics must be regulated strictly in a spatiotemporal manner for proper biological output. This is achieved through functional interaction with other cytoskeletal components, intermediate filaments (IFs) and microtubules (MTs), and hundreds of accessory proteins. Signaling factors that stimulate F-actin rearrangement such as extracellular matrices (ECMs) and growth factors are known to cause an increase in Abl tyrosine kinase activity, followed by the relocalization of Abl to specific F-actin structures such as focal adhesion sites, lamellipodia, and membrane ruffles depending on the stimulus [
How can Abl be involved in such diverse aspects of actin dynamics? Primary as well as higher structural features may underlie such functional properties. Indeed, the overall domain structure of the Abl kinases is unique among the tyrosine kinase family [
In this paper, we first introduce structural and functional characteristics of Abl, summarize the outline of Abl-mediated F-actin signaling cascades, and subsequently describe the roles and functional modes of an adaptor protein, Abi, in the process of Abl-mediated actin dynamics, in comparison with those of another adaptor protein, Crk.
The Abl gene was first isolated as a transforming gene in the Abelson murine leukemia virus (A-MuLV) [
Abl and Arg seem to have substantial functional overlap
The overall domain structure of each Abl family protein is shown in Figure
Primary structure of Abl kinases. Only isoform 1b of c-Abl and Arg in mice is shown. Both have a “Cap” region at the extreme N-terminus (Cap, gray), followed by the Src homology-3 domain (SH3, orange), Src homology-2 domain (SH2, blue-green), and catalytic domain (kinase, purple). However, at the extreme N-terminus, v-Abl contains a viral “Gag” sequence (Gag, green) by replacing the “Cap” and the SH3, and Bcr-Abl contains the N-terminal portion of the Bcr protein (Bcr, magenta) by replacing the “Cap,” which were generated by uptake into a retrovirus and chromosomal translocation, respectively. In the C-terminal half, Abl has four P-x-x-P motifs and Arg has three in the proline-rich region (PR, pink). At the extreme C-terminus, there are actin-binding domains in Abl and Arg (AB, beige). In addition, Abl has a DNA-binding region (DB, light blue), while Arg has a microtubule-binding domain (MT, blue). Amino acid residues of each gene in mice are numbered except for the human P210 Bcr-Abl gene [
Abl kinase activity is regulated in a variety of ways reflecting its domain structure, and this regulation may confer on Abl its role as a mediator of signaling in actin dynamics [
Signaling upstream and downstream of Abl leading to actin dynamics. Abl kinase activity, localization, and substrate phosphorylation which are responsible for F-actin dynamics may be modulated by ligand molecules, accessory proteins, phosphorylation by other kinases, and/or adaptor proteins as described in the text. However, the role of adaptor proteins is mainly highlighted in this paper. ECM: extracellular matrix.
The importance of the C-terminal half in the regulation of Abl has been clarified in various studies [
Adaptor proteins for c-Abl in mice.
Adaptor protein | Target domain in Abl | References |
---|---|---|
c-Crk : CrkI, CrkII | Proline-rich motif | [ |
CrkL | Proline-rich motif | [ |
Abi1/E3B1 | Proline-rich motif/SH3 | [ |
Abi2/ArgBP1 | Proline-rich motif/SH3 | [ |
CAP/Ponsin/SH3P12 | Proline-rich motif | [ |
Vinexin | Proline-rich motif | [ |
Nck | Proline-rich motif | [ |
Cbl | Proline-rich motif | [ |
ArgBP2/Sorbs2 | Proline-rich motif | [ |
Adaptor proteins that bind Abl and function in Abl-mediated signaling in mouse cells are listed together with the target domain in Abl.
Nine proteins have been identified in mice that bind the C-terminal half of Abl and function as adaptors (Table
The Abi family proteins, Abi1 and Abi2, were originally identified as proteins that bind to the P-x-x-P motif in Abl and negatively modulate its transforming activity [
Domain structure of the Abi and Crk family proteins. (a) Modular domains of the Abi and Crk family proteins in mice are illustrated. For the Abi family, WAB (light green): WAVE-binding domain, SNARE (grey): Syntax-1 binding domain which is an overlapping domain of WAB, HHR (red): Hox homology region, SR: serine/threonine-rich region, PP (cyan): polyproline structure, SH3 (orange): SH3 domain [
Abi1-null mice die in the midgestational stage and show phenotypes similar to those of
Abi1 and Abi2 play a dual role as potential effectors and regulators of the Abl kinase [
Regulatory mechanism of Abl kinase activity by Abi and Crk. (a) Events leading to the activation of the Abl kinase activity after the interaction of Abl and Abi1 or Abi2 are schematically illustrated. The regulatory Y213 and Y398 in Abi1 and Y324 in Abi2 are indicated. Phosphorylated Y213 and Y398 in Abi1 are involved in the interaction with c-Abl through the SH2 and PR region of c-Abl, respectively. (b) Y221 and Y251 in CrkII are sites of phosphorylation by Abl after the interaction of Abl and CrkII. When Y221 is phosphorylated, it induces an “autoinhibition” through an intramolecular SH2-pTyr clamp and the suppression of Abl kinase activity, resulting in the disassembly of Crk-mediated signaling complexes and abrogation of Crk-mediated signaling. On the other hand, Y251 in the SH3 domain interacts with the SH2 domain of Abl and releases the latch and clamp structure of Abl, leading to the activation of Abl.
The modulation of Abl kinase by Crk occurs in discrete steps [
The interaction of Abi and Crk with Abl is also regulated by the phosphorylation of Abl by other protein kinases. For example, the serine/threonine kinase Pak2 phosphorylates c-Abl at S637 and S638 which reside next to the P-x-x-P motif [
Abl transduces extracellular signals to cytoskeleton actin assembly/disassembly through its kinase activity. The actin-related protein-2/3 (Arp2/3) complex is a central player in the regulation of both the initiation of actin polymerization and the organization of the resulting filaments [
Mena (mammalian-enabled)/VASP (vasodilator-stimulated phosphoprotein) proteins interact with barbed ends of actin-filament and permit its elongation by preventing the capping of the barbed end: the anticapping hypothesis [
Cells achieve cell-cell adhesion through cadherin receptors which are linked to the actin cytoskeleton through the catenin complex and are regulated by the Rho GTPase family [
Adaptor proteins not only are important for the regulation of Abl as described above, but also regulate the downstream signaling machineries leading to acitn dynamics. Abi and Crk proteins bind Abl and also function as scaffold proteins that permit the assembly of respective multimolecular complexes (Figure
Furthermore, Abi1 connects Abl and the substrate. Abi1 binds Mena and VASP through the polyproline region of Abi (Figure
Bcr-Abl-positive leukemic cells exhibit abnormalities in cell motility, cell adhesion, and integrin function [
Crk, on the other hand, binds and activates Abl kinase as described above, inducing increased tyrosine phosphorylation of p130Cas [
While the integrin-induced tyrosine phosphorylation of paxillin and p130Cas [
Finally, upstream signals and downstream signaling cascades of Abl-mediated actin dynamics are schematically summarized in Figure
Differential roles of Abi and Crk in signaling pathways of Abl-mediated actin dynamics. Upstream and downstream signaling cascades of Abl-mediated actin dynamics are schematically illustrated (solid arrow). Downstream signaling pathways unique to each member of the Abi and Crk families are also shown together with the respective outcomes (broken arrow).