ASAP1 is an Arf GTPase activating protein (GAP) with activity towards Arf1 and Arf5 but not Arf6 However, it has been shown to bind GTP-Arf6 stably without GAP activity []. It has been implicated in cell growth, migration, and survival, as well as in tumor invasion and malignancy. It binds paxillin and cortactin, two components of invadopodia which are essential for tumor invasiveness. It also binds focal adhesion kinase (FAK) and the SH2/SH3 adaptor CrkL [, ]. ASAP1 contains an N-terminal BAR domain, followed by a Pleckstrin homology (PH) domain, an Arf GAP domain, ankyrin (ANK) repeats, and a C-terminal SH3 domain [].This entry represents the BAR domain of ASAP1. BAR domains form dimers that bind to membranes, induce membrane bending and curvature, and may also be involved in protein-protein interactions. The BAR domain of ASAP1 mediates membrane bending, is essential for function, and autoinhibits GAP activity by interacting with the PH and/or Arf GAP domains [].
The IQSEC (IQ motif and SEC7 domain-containing protein), also called BRAG (Brefeldin A-resistant Arf-gunanine nucleotide exchange factor) family are a subset of Arf GEFs that have been shown to activate Arf6, which acts in the endocytic pathway to control the trafficking of a subset of cargo proteins including integrins. IQSEC family also have key roles in the function and organization of distinct excitatory and inhibitory synapses in the retina [, ]. The family consists of 3 members: IQSEC1 (also called BRAG2/GEP100), IQSEC2 (also called BRAG1), and IQSEC3 (also called SynArfGEF, BRAG3, or KIAA1110). IQSEC1 interacts with clathrin and modulates cell adhesion by regulating integrin surface expression and in addition to Arf6, it also activates the class II Arfs, Arf4 and Arf5 []. Mutations in IQSEC2 cause non-syndromic X-linked intellectual disability as well as reduced activation of Arf substrates (Arf1, Arf6) []. IQSEC3 regulates Arf6 at inhibitory synapses and associates with the dystrophin-associated glycoprotein complex and S-SCAM [].These IQSEC family members contain a IQ domain that may bind calmodulin, a PH domain that is thought to mediate membrane localization, and a SEC7 domain that can promote GEF activity on ARF []. The PH domain of BRAG2 has been shown to potentiate nucleotide exchange by binding to polyanionic membranes without marked phosphoinositides preference [].
Arf GTPases are involved in the formation of coated carrier vesicles by recruiting coat proteins. This entry includes Arf1, Arf2, Arf3, Arf4, Arf5, and related proteins. Each contains an N-terminal myristoylated amphipathic helix that is folded into the protein in the GDP-bound state. GDP/GTP exchange exposes the helix, which anchors to the membrane. Following GTP hydrolysis, the helix dissociates from the membrane and folds back into the protein. A general feature of Arf1-5 signaling may be the cooperation of two Arfs at the same site. Arfs1-5 are generally considered to be interchangeable in function and location, but some specific functions have been assigned []. Arf1 localizes to the early/cis-Golgi, where it is activated by GBF1 and recruits the coat protein COPI. It also localizes to the trans-Golgi network (TGN), where it is activated by BIG1/BIG2 and recruits the AP1, AP3, AP4, and GGA proteins []. Humans, but not rodents and other lower eukaryotes, lack Arf2. Human Arf3 shares 96% sequence identity with Arf1 and is believed to generally function interchangeably with Arf1. Human Arf4 in the activated (GTP-bound) state has been shown to interact with the cytoplasmic domain of epidermal growth factor receptor (EGFR) and mediate the EGF-dependent activation of phospholipase D2 (PLD2), leading to activation of the activator protein 1 (AP-1) transcription factor []. Arf4 has also been shown to recognise the C-terminal sorting signal of rhodopsin and regulate its incorporation into specialised post-Golgi rhodopsin transport carriers (RTCs) []. There is some evidence that Arf5 functions at the early-Golgi and the trans-Golgi to affect Golgi-associated alpha-adaptin homology Arf-binding proteins (GGAs) [].
