Arf GAPs: A family of proteins with disparate functions that converge on a common structure, the integrin adhesion complex

Abstract

ADP-ribosylation factors (Arfs) are members of the Ras GTPase superfamily. The function of Arfs is dependent on GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs), which allow Arfs to cycle between the GDP-bound and GTP-bound forms. Arf GAPs have been shown to be present in integrin adhesion complexes, which include focal adhesions. Integrin adhesion complexes are composed of integrins, scaffolding proteins and signaling proteins and regulate cell proliferation, survival, differentiation and migration. Understanding the role of Arf GAPs in the regulation of integrin adhesion complexes is relevant to understanding normal physiology and cancer. In this review, we will discuss the contribution of the Arf GAP family members to the regulation of integrin adhesion complexes, examining the diverse mechanisms by which they control integrin adhesion complex formation, maturation and dissolution. GIT1 and ARAP2 serve as GAPs for Arf6, regulating Rac1 and other effectors by mechanisms still being defined. In contrast, GIT2 regulates Rac1 independent of Arf6. AGAP2 binds to and regulates focal adhesion kinase (FAK). ARAP2 and ACAP1, both Arf6 GAPs, regulate membrane trafficking of integrins through different endocytic pathways, exerting opposite effects on focal adhesions. ASAP1 not only regulates actin cytoskeleton remodeling through its interaction with nonmuscle myosin 2A, but is also important in integrin recycling. These examples illustrate the diversity and versatility of Arf GAPs as regulators of integrin adhesion complex structure and function.

Keywords: ADP-ribosylation factor GTPase-activating protein; ADP-ribosylation factors; ARAP2; ASAP1; GIT1/2; Integrin adhesion complex.

Figure 1.

A. Schematic of Arf GAPs involved in regulation of integrin adhesion complexes. Domain structures of GIT1, GIT2, ASAP1, ASAP3, ACAP1, ARAP2 and AGAP2. A, Ankyrin repeats; ArfGAP, Arf GTPase activating domain; BAR, Bin/Amphiphysin/Rvs domain; CC, coiled-coiled motif; (D/ELPPKP)8, 8 tandem proline-rich (D/ELPPKP) motifs; GLD, GTP-binding protein-like domain; PBS, paxillin binding site; PH, pleckstrin homology domain; PPXPP, cluster of 3 proline-rich motifs; Rho GAP, Rho GTPase activating domain; S, serine rich; SAM, sterile α-motif; SH3, Src homology 3 domain; SHD, Spa homology domain.

Figure 2.

Roles of Arf GAPs in regulation of integrin adhesion complexes. The cartoon illustrates 3 mechanisms by which Arf GAPs can affect integrin adhesion complexes and the potential sites of action of the Arf GAPs in cells. First is by signaling through Rac1 and through phosphorylation (red arrows). Rac1•GTP blocks the transition of nascent adhesions and focal complexes to focal adhesions. Arf6•GTP facilitates Rac1 activation. GIT1 and ARAP2 reduce Rac1•GTP levels by reducing Arf6•GTP levels through GAP activity. GIT2 decreases Rac1•GTP levels by a Crk-dependent mechanism not yet defined (indicated by a question mark) that does not involve Arf6. Phosphorylation of FAK and FAK kinase activity also control integrin adhesion dynamics. Change in phosphorylation of FAK consequent to overexpression of AGAP2 has been found to reduce stability of integrin adhesions. The second mechanism by which Arf GAPs affect integrin adhesion complexes is by control of membrane trafficking of integrins (blue). This mechanism is used by at least 4 Arf GAPs: ACAP1, ARAP2, ASAP1 and ASAP3. ACAP1 functions on a tubulovesicular recycling compartment containing Arf6•GTP and Arf6•GTP is required for integrin recycling from this compartment. ARAP2 functions with Arf6 at a distinct endocytic compartment involved in the trafficking of integrins. ASAP1 and ASAP3 affect recycling of integrins but the specific compartment has not been defined. Furthermore, a role for GAP activity of ASAP1 and ASAP3 in integrin recycling has not been established nor whether ASAP3 is inhibitory or stimulatory. The third mechanism is by control of actomyosin, used by ASAP1, which directly binds to nonmuscle myosin 2A (NM2A, green).