Proteomics identifies ubiquitin-proteasome targets and new roles for chromatin-remodeling in the Arabidopsis response to phosphate starvation.

In order to identify new regulators of the phosphate (Pi) starvation signaling pathway in plants, we analyzed variation in the abundance of nuclear-enriched proteins isolated from Arabidopsis roots that depends on Pi supply. We used 2-D fluorescence difference gel electrophoresis and MALDI-TOF/TOF techniques for proteome separation, visualization and relative protein abundance quantification and identification. Pi-controlled proteins identified in our analysis included components of the chromatin remodeling, DNA replication, and mRNA splicing machineries. In addition, by combining Pi starvation conditions with proteasome inhibitor treatments, we characterized the role of the ubiquitin-proteasome system, a major mechanism for targeted protein degradation in eukaryotes, in the control of the stability of Pi-responsive proteins. Among Pi-responsive proteins, the histone chaperone NAP1;2 was selected for further characterization, and was shown to display differential nucleo-cytoplasmic accumulation in response to Pi deprivation. We also found that mutants affecting three members of the NAP1 family accumulate lower Pi levels and display reduced expression of Pi starvation-inducible genes, reflecting a potential regulatory role for these chromatin-remodeling proteins in Pi homeostasis. BIOLOGICAL SIGNIFICANCE: In this study, we explore the feasibility of nuclear proteomics to identify regulatory proteins and ubiquitin-proteasome targets within a specific stress signaling pathway in plants, in our case phosphate starvation signaling in Arabidopsis. It will be of interest for researchers involved in the dissection of any signaling pathway in plants, in particular those with an interest in the ubiquitin-proteasome functions, and for the plant nutrition community.