1. Max Planck Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany

Correspondence to: Antje Berken¹ Correspondence and requests for materials should be addressed to A.B. (Email: antje.berken@mpi-dortmund.mpg.de). Accession numbers for the plant RopGEF proteins in Arabidopsis thaliana, Oryza sativa, Medicago truncatula and Lycopersicon esculentum are listed in Fig. 2.

Abstract

In plants, the small GTP-binding proteins called Rops work as signalling switches that control growth, development and plant responses to various environmental stimuli^{1, 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 conformation^{4, 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 elusive^{1, 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.

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