RT Journal Article SR Electronic T1 Inositol pyrophosphates promote the interaction of SPX domains with the coiled-coil motif of PHR transcription factors to regulate plant phosphate homeostasis JF bioRxiv FD Cold Spring Harbor Laboratory SP 2019.12.13.875393 DO 10.1101/2019.12.13.875393 A1 Martina K. Ried A1 Rebekka Wild A1 Jinsheng Zhu A1 Larissa Broger A1 Robert K. Harmel A1 Ludwig A. Hothorn A1 Dorothea Fiedler A1 Michael Hothorn YR 2019 UL http://biorxiv.org/content/early/2019/12/13/2019.12.13.875393.abstract AB Phosphorus is an essential nutrient taken up by organisms in the form of inorganic phosphate (Pi). Eukaryotes have evolved sophisticated Pi sensing and signalling cascades, enabling them to maintain cellular Pi concentrations. Pi homeostasis is regulated by inositol pyrophosphate signalling molecules (PP-InsPs), which are sensed by SPX-domain containing proteins. In plants, PP-InsP bound SPX receptors inactivate Myb coiled-coil (MYB-CC) Pi starvation response transcription factors (PHRs) by an unknown mechanism. Here we report that a InsP8 – SPX complex targets the plant-unique CC domain of PHRs. Crystal structures of the CC domain reveal an unusual four-stranded anti-parallel arrangement. Interface mutations in the CC domain yield monomeric PHR1, which is no longer able to bind DNA with high affinity. Mutation of conserved basic residues located at the surface of the CC domain disrupt interaction with the SPX receptor in vitro and in planta, resulting in constitutive Pi starvation responses. Together, our findings suggest that InsP8 regulates plant Pi homeostasis by controlling the oligomeric state and hence the promoter binding capability of PHRs via their SPX receptors. (173 words)