Mechanisms underlying the specificity of action of ARF guanine-nucleotide exchange factor GNOM in polar recycling in Arabidopsis

• Applicant: Professor Dr. Gerd Jürgens
• Subject Area: Plant Cell and Developmental Biology
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 574845372

ARF guanine-nucleotide exchange factors such as GNOM play a decisive role in membrane trafficking by activating small GTPases of the ARF family through GDP-GTP exchange, which enables the formation of transport vesicles. On the one hand, GNOM is a member of the GBF1 family which mediates retrograde transport of COPI vesicles from Golgi stacks to the ER. On the other hand, GNOM has a specialized primary role, mediating polar recycling of auxin efflux carrier PIN1 from endosomes to the basal plasma membrane and thus directional auxin transport. Moreover, GNOM can replace closely related ARF-GEF GNOM-LIKE1 (GNL1) which normally takes on retrograde Golgi-ER transport. Conversely, GNL1 cannot substitute for GNOM in polar recycling. During plant evolution, these two proteins evolved from an ancestral protein that performed both tasks. In this proposal, we will identify mechanisms of how GNOM is targeted to endosomes whereas GNL1 can only associate with Golgi membranes, and how the primary action of GNOM results in polar recycling of PIN1. Based on previous findings, we propose to identify important protein sequences for endosomal association by functional analysis of GNOM-GNL1 chimeric proteins. In this way, we will narrow down relevant segments and eventually identify critical amino-acid motifs. In addition, we will address how GNOM associates with endosomes, by direct binding or via "docking proteins". In this context, we will carry out protein-protein interaction screens. Regarding the action of GNOM in polar ecycling, we will predominantly functionally characterize a GNOM-specific interactor named PRAF or RLD and its known interactors to assess whether and how they might relay the primary action of GNOM. We will also analyse other PRAF interactors that we identified. If no known interactor relays GNOM action we will perform protein-protein interaction screens to identify additional PRAF/RLD interactors and characterise the new candidates.

• DFG Programme: Research Grants