Correct targeting of plant ARF GTPases relies on distinct protein domains.

Indispensable membrane trafficking events depend on the activity of conserved small guanosine triphosphatases (GTPases), anchored to individual organelle membranes. In plant cells, it is currently unknown how these proteins reach their correct target membranes and interact with their effectors. To address these important biological questions, we studied two members of the ADP ribosylation factor (ARF) GTPase family, ARF1 and ARFB, which are membrane anchored through the same N-terminal myristoyl group but to different target membranes. Specifically, we investigated how ARF1 is targeted to the Golgi and post-Golgi structures, whereas ARFB accumulates at the plasma membrane. While the subcellular localization of ARFB appears to depend on multiple domains including the C-terminal half of the GTPase, the correct targeting of ARF1 is dependent on two domains: an N-terminal ARF1 domain that is necessary for the targeting of the GTPase to membranes and a core domain carrying a conserved MxxE motif that influences the relative distribution of ARF1 between the Golgi and post-Golgi compartments. We also established that the N-terminal ARF1 domain alone was insufficient to maintain an interaction with membranes and that correct targeting is a protein-specific property that depends on the status of the GTP switch. Finally, an ARF1-ARFB chimera containing only the first 18 amino acids from ARF1 was shown to compete with ARF1 membrane binding loci. Although this chimera exhibited GTPase activity in vitro, it was unable to recruit coatomer, a known ARF1 effector, onto Golgi membranes. Our results suggest that the targeting of ARF GTPases to the correct membranes may not only depend on interactions with effectors but also relies on distinct protein domains and further binding partners on the Golgi surface.