Coordinated Activation of ARF1 GTPases by ARF-GEF GNOM Dimers Is Essential for Vesicle Trafficking in Arabidopsis.

Membrane traffic maintains the organization of the eukaryotic cell and delivers cargo proteins to their subcellular destinations such as sites of action or degradation. Membrane vesicle formation requires ARF GTPase activation by the SEC7 domain of ARF guanine-nucleotide exchange factors (ARF-GEFs), resulting in the recruitment of coat proteins by GTP-bound ARFs. In vitro exchange assays were done with monomeric proteins, although ARF-GEFs form dimers in vivo. This feature is conserved across the eukaryotes, however its biological significance is unknown. Here we demonstrate FRET-FLIM detectable proximity of ARF1•GTPs in vivo and we show that this is mediated through coordinated activation by ARF-GEF dimers such as Arabidopsis GNOM involved in polar recycling of PIN1. Mutational disruption of ARF1 spacing as detected by FRET-FLIM interfered with ARF1-dependent trafficking but not coat protein recruitment in Arabidopsis. A mutation impairing the interaction of one of the two SEC7 domains of GNOM ARF-GEF dimer with its ARF1 substrate reduced the efficiency of coordinated ARF1 activation. Our results suggest a model of coordinated activation-dependent membrane insertion of ARF1•GTP molecules required for coated membrane vesicle formation. Considering the evolutionary conservation of ARFs and ARF-GEFs, this initial regulatory step of membrane trafficking might well occur in eukaryotes in general.