Impact of C-terminal truncations in the Arabidopsis Rab escort protein (REP) on REP-Rab interaction and plant fertility.

Lipid anchors are common post-translational modifications for proteins engaged in signaling and vesicular transport in eukaryotic cells. Rab proteins are geranylgeranylated at their C-termini, a modification which is important for their stable binding to lipid bilayers. The Rab escort protein (REP) is an accessory protein of the Rab geranylgeranyl transferase (RGT) complex and it is obligatory for Rab prenylation. While REP-Rab interactions have been studied by biochemical, structural, and genetic methods in animals and yeast, data on the plant RGT complex are still limited. Here we use hydrogen-deuterium exchange mass spectrometry (HDX-MS) to describe the structural basis of plant REP-Rab binding. The obtained results show that the interaction of REP with Rabs is highly dynamic and involves specific structural changes in both partners. In some cases the Rab and REP regions involved in the interaction are molecule-specific, and in other cases they are common for a subset of Rabs. In particular, the C-terminus of REP is not involved in binding of unprenylated Rab proteins in plants, in contrast to mammalian REP. In line with this, a C-terminal REP truncation does not have pronounced phenotypic effects in planta. On the contrary, a complete lack of functional REP leads to male sterility in Arabidopsis: pollen grains develop in the anthers, but they do not germinate efficiently and hence are unable to transmit the mutated allele. The presented data show that the mechanism of action of REP in the process of Rab geranylgeranylation is different in plants than in animals or yeast.