ARF6

ARF6 Full Name

ADP-ribosylation factor 6

ARF6 Introduction

ARF6 (ADP ribosylation factor 6) is a member of the ARF family of small GTPases within the Ras superfamily. The gene is located on chromosome 14q21.3 and encodes a protein of 175 amino acids with a molecular weight of approximately 20 kDa. Unlike ARF1 which primarily functions at the Golgi apparatus, ARF6 is localized to the plasma membrane and endosomal compartments, where it plays critical roles in the regulation of endocytosis, exocytosis, actin cytoskeleton remodeling, membrane recycling, and cell migration. ARF6 cycles between an inactive GDP-bound form (cytosolic) and an active GTP-bound form (membrane-associated). Guanine nucleotide exchange factors (GEFs) promote the activation of ARF6, while GTPase-activating proteins (GAPs) stimulate GTP hydrolysis to inactivate it. In its active form, ARF6 regulates the trafficking of specific cargo between the plasma membrane and endosomal compartments, particularly the recycling of integrins, cadherins, and growth factor receptors. ARF6 also controls the formation of actin-rich membrane protrusions such as lamellipodia and filopodia, positioning it as a master regulator of cell adhesion, spreading, and migration. ARF6 is ubiquitously expressed, with particularly high levels in immune cells, endothelial cells, neurons, and cancer cells. Dysregulation of ARF6 has been implicated in cancer progression, immune disorders, neurological diseases, and infectious diseases.

Figure 1. Schematic structure of ARF6.

Roles in Endosomal Recycling and Actin Remodeling
ARF6 is a master regulator of the endosomal recycling pathway, controlling the return of internalized cargo from endosomes back to the plasma membrane. Upon activation by ARF6 GEFs including ARNO (ARF nucleotide-binding site opener, also known as CYTH2), EFA6 (exchange factor for ARF6), and BRAG1, ARF6-GTP promotes the formation of recycling endosomes and the sorting of specific cargo into these structures. ARF6 regulates the recycling of multiple cargo types. It controls the recycling of integrins (αvβ3, α5β1, αLβ2), which are critical for cell adhesion to the extracellular matrix, promoting cell spreading and migration. ARF6 also regulates the recycling of cadherins, particularly E-cadherin, which is essential for cell-cell adhesion maintenance. ARF6 controls the recycling of growth factor receptors, including the EGF receptor, leading to sustained signaling. In addition, ARF6 regulates the recycling of major histocompatibility complex class I (MHC-I) and other immune receptors. Beyond its role in recycling, ARF6 regulates the actin cytoskeleton through multiple effectors. ARF6 activates Rac1 and RhoG through the ELMO/DOCK180 complex, leading to the formation of lamellipodia and membrane ruffles. ARF6 also activates phospholipase D (PLD), which produces phosphatidic acid, a lipid second messenger that promotes actin polymerization. ARF6 regulates the activity of the Arp2/3 complex, which nucleates branched actin filaments, and of formins, which promote linear actin polymerization. These actin regulatory functions enable ARF6 to control cell migration, invasion, phagocytosis (in macrophages), and neurite outgrowth (in neurons). ARF6 also regulates membrane tension, clathrin-mediated endocytosis, caveolar endocytosis, and the formation of exosomes.

Regulation and Therapeutic Implications
The activity of ARF6 is tightly regulated by the opposing actions of GEFs and GAPs. ARF6 GEFs include the ARNO/CYTH family (ARNO/CYTH2, CYTH1, CYTH3), the EFA6 family (EFA6A-D, also known as PSD proteins), and the BRAG family (BRAG1, BRAG2, BRAG3). These GEFs are regulated by phosphoinositides (particularly PIP3), by phosphorylation, and by protein-protein interactions. ARF6 GAPs include the ACAP family (ACAP1, ACAP2, ACAP3), the ARFGAP1/2/3 family, and the SMAP family (SMAP1, SMAP2). These GAPs inactivate ARF6 and are themselves regulated by phosphoinositides and by interacting proteins. ARF6 activity is also regulated by post-translational modifications. N-terminal myristoylation is essential for membrane association. ARF6 is phosphorylated at Tyr-46 by Src family kinases, which enhances its activity and promotes cancer cell invasion. ARF6 is also phosphorylated at Thr-44 by protein kinase A (PKA) and at Ser-164 by casein kinase 2 (CK2), modifications that regulate its activity and interactions. Therapeutically, ARF6 is an attractive target for cancer treatment, given its role in migration, invasion, metastasis, and therapy resistance. Several classes of ARF6 inhibitors are being developed. NAV-2729 is a small molecule that inhibits ARF6 activation by blocking the interaction between ARF6 and its GEFs, and has shown efficacy in preclinical models of breast cancer, melanoma, and pancreatic cancer. Other ARF6 inhibitors include ARF6-selective peptides and PROTACs (proteolysis-targeting chimeras) that degrade ARF6.

Alternate Names for ARF6

ARF6; ADP-ribosylation factor 6