Arf GAPs as Regulators of the Actin Cytoskeleton-An Update

Abstract

Arf GTPase-activating proteins (Arf GAPs) control the activity of ADP-ribosylation factors (Arfs) by inducing GTP hydrolysis and participate in a diverse array of cellular functions both through mechanisms that are dependent on and independent of their Arf GAP activity. A number of these functions hinge on the remodeling of actin filaments. Accordingly, some of the effects exerted by Arf GAPs involve proteins known to engage in regulation of the actin dynamics and architecture, such as Rho family proteins and nonmuscle myosin 2. Circular dorsal ruffles (CDRs), podosomes, invadopodia, lamellipodia, stress fibers and focal adhesions are among the actin-based structures regulated by Arf GAPs. Arf GAPs are thus important actors in broad functions like adhesion and motility, as well as the specialized functions of bone resorption, neurite outgrowth, and pathogen internalization by immune cells. Arf GAPs, with their multiple protein-protein interactions, membrane-binding domains and sites for post-translational modification, are good candidates for linking the changes in actin to the membrane. The findings discussed depict a family of proteins with a critical role in regulating actin dynamics to enable proper cell function.

Keywords: ADP-ribosylation factor GTPase-activating protein; ADP-ribosylation factors; ARAP1; ARAP2; ASAP1; GIT1/2; actin; focal adhesion; phagocytosis; podosome.

Figure 1

The domain structures of Arf GAPs. Representative domain organizations of each human Arf GAP group are depicted but not to scale. The number in parentheses below the name of the subfamily indicates the number of members in each Arf GAP subfamily. Arf GAP, Arf GTPase-activating domain; ALPS, ArfGAP1 lipid-packing sensor; CB, clathrin-box; CALM, CALM binding domain; FG repeats, multiple copies of the XXFG motif; PH, pleckstrin homology domain; A, ankyrin repeat; PBS, Paxillin binding site; SHD, Spa-homology domain; CC, coiled-coil; BAR, Bin/Amphiphysin/Rvs; cluster of three Proline-rich (PxxP) motifs; (E/DLPPKP)8, eight tandem (E/DLPPKP) motifs; SH3, Src homology 3 domain; SAM, sterile α-motif; RhoGAP, RhoGAP domain; RA, Ras association motif; GLD, GTP-binding protein-like domain. SMAP2 has CALM, but SMAP1 does not. ASAP1 contains the Pro (E/DLPPKP) repeat but ASAP2 and ASAP3 do not. ASAP3 lacks an SH3 domain.

Figure 2

The roles of ASAP1, GITs and ARAP2 in the regulation of actin structures involved in lamellipodia-dependent cell migration. The cartoon illustrates the pathways that ASAP1, GITs and ARAP2 control to modulate the antagonistic relationship between lamellipodia formation and stress fiber formation/focal adhesion (FA) maturation. Arrows indicate positive/stimulatory regulation while ⊥ indicates negative/inhibitory regulation. Double arrows indicate binding. Dash lines denote possible regulation and the question mark denotes ASAP1 may function upstream or downstream from RhoA.

Figure 3

The modes of action that Arf GAPs take to regulate actin and related functions. Arf GAPs regulate actin-based structures and functions by acting like (1) Arf effectors that bind Arf•GTP, propagate and promote Arf functions independent of GAP activity; (2) Arf cyclers that, by working with Arf GEFs, facilitate the cycling of Arfs between GTP and GDP bound states to aid dynamic assembly and disassembly of actin structures, and (3) Arf terminators that reduce Arf•GTP levels and terminate consequent signaling for actin remodeling. Examples for each mode of action are shown. ARAP2* denotes that parts of ARAP2 function in focal adhesions (FAs) are mediated by other mechanisms. + and − mean “promoting” and “inhibiting” respectively.