Phosphorylation of RADICAL-INDUCED CELL DEATH1 by Photoregulatory Protein Kinases

RADICAL-INDUCED CELL DEATH1 (RCD1) is a multidomain protein with intrinsically disordered regions (IDRs) separating its highly structured domains. RCD1 regulates plant development and stress tolerance by interacting with numerous transcription factors (TFs) through its C-terminal RST domain. It has been shown to interact in a non-RST-dependent manner with Photoregulatory Protein Kinases (PPKs). In Arabidopsis, this recently discovered family of protein kinases is comprised of four members that localize to nuclear bodies (NBs), subnuclear non-membrane bound complexes of mostly unknown function. PPK-dependent phosphorylation has been shown to target proteins for degradation, thereby playing an essential role in protein turnover. Mass spectrometric analysis of the in vitro phosphorylated GST-RCD1 revealed several PPK-dependent RCD1 phosphopeptides that were also identified in the in vivo pull-down experiments. Most of these phosphosites were clustered in the IDR2, the intrinsically disordered region between the WWE and PARP-like domains. Using transgenic lines expressing non-phosphorylatable version of RCD1 we showed that phosphorylation of the IDR2 is involved in the regulation of the RCD1 affecting its subnuclear distribution to NBs of different size and number. Moreover, phosphorylation of IDR2 facilitates degradation of RCD1 and possibly also its partner TFs. Thus, it is necessary for the function of RCD1 as a negative transcriptional co-regulator.