Endosome-specific localization and function of the ARF activator GNOM.

ARF GTPases act at multiple steps of the secretory and vacuolar/lysosomal trafficking pathways, but little is known about the spatial regulation of ARF activation. In this issue of Cell, Geldner et al. demonstrate that the Arabidopsis ARF activator GNOM localizes to endosomes where it controls the polarized trafficking of the auxin efflux carrier PIN1 to the basal plasma membrane.