Introduction

The mammalian ADP-ribosylation factor (Arf) subfamily of Ras-related small G-proteins was originally named for the ability to stimulate cholera toxin mediated ADP-ribosylation of the Gsα subunit utilized by many GPCRs (1).  The Arf GTPases have been grouped into three classes based on their size and amino acid similarity (2): class I (Arf6 and Arf3), class II (Arf4 and Arf5) and class III (Arf6).  One of the primary roles of Arf6 is in protein trafficking  from the plasma membrane to endocytic compartments (e.g., GPCRs) and appears to have a role in the anterograde transport of some GPCRs (3, 4).  Like Arf1, Arf6 functions in part by the activation of lipid modifying enzymes that alter the local membrane environment  (3).

Arf6, like other small G-proteins, cycles between the inactive GDP-bound and the active GTP-bound states.  The preferential association of effector proteins with the GTP-bound over the GDP-bound state of Arf6 provides the basis for Arf6’s function in the cell.  This highly specific association of effector proteins with Arf6-GTP has been exploited to develop affinity precipitation assays to monitor Arf6 activation (5).

Cytoskeleton’s Arf6 Activation Assay Biochem Kit? utilizes the Arf6 protein binding domain (PBD) of the effector protein GGA3 (Golgi-localized γ-ear containing, Arf-binding protein 3), which has been shown to specifically bind the GTP-bound form of Arf6 (5, 6).  We have covalently conjugated purified GGA3-PBD (amino acids 1-316) expressed in E. coli to the colored sepharose beads provided in this kit (Figure 1).  Using these beads, the researcher is able to “pull-down” Arf6-GTP and quantify the level of active Arf6 with a subsequent Western blotting step using the Arf6 specific antibody provided in this kit. This assay provides a simple means of analyzing cellular Arf6 activation levels in a variety of systems.  A typical Arf6 pull-down assay is shown in Figure 2 below using either GTPγS and GDP loaded MDCK cell extracts or extracts from MDCK cells that have been harvested and held in suspension or were plated onto fibronectin coated culture dishes (e.g., see references 7, 8).