A new family of RhoGEFs activates the Rop molecular switch in plants

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Abstract
In plants, the small GTP-binding proteins called Rops work as signalling switches that control growth, development and plant responses to various environmental stimuli1,2,3. Rop proteins (Rho of plants, Rac-like and AtRac in Arabidopsis thaliana) belong to the Rho family of Ras-related GTP-binding proteins that turn on signalling pathways by switching from a GDP-bound inactive to a GTP-bound active conformation4,5. Activation depends on guanine nucleotide exchange factors (GEFs) that catalyse the otherwise slow GDP dissociation for subsequent GTP binding6. Although numerous RhoGEFs exist in animals and yeasts7, no Rop-specific GEFs have yet been identified in plants and so Rop activation has remained elusive1,2,3,8. Here we describe a new family of RhoGEF proteins that are exclusive to plants. We define a unique domain within these RopGEFs, termed PRONE (plant-specific Rop nucleotide exchanger), which is exclusively active towards members of the Rop subfamily. It increases nucleotide dissociation from Rop more than a thousand-fold and forms a tight complex with nucleotide-free Rop. RopGEFs may represent the missing link in signal transduction from receptor kinases to Rops and their identification has implications for the evolution of the Rho molecular switch.