Protein Phosphatase 1 controls function of Ikaros transcriptional factor and regulates chromatin remodeling during lymphocyte differentiation

Abstract

Ikaros encodes a zinc finger protein that is essential for lymphoid development. Ikaros protein binds to the upstream regulatory elements (URE) of target genes and regulates their transcription via chromatin remodeling. We have reported that the dephosphorylation of Ikaros by protein phosphatase 1 (PP1) regulates Ikaros’ subcellular localization and protein stability. (Popescu et al. J Biol Chem 2009 284:13869). We studied the effect of PP1 on Ikaros function in primary thymocytes. Inhibition of PP1 activity abolishes the repression of Ikaros target genes during thymocyte differentiation. Co‐immunoprecipitation experiments show that Ikaros phosphomimetic mutants and Ikaros protein with a point mutation at the PP1‐interaction site: 1) do not associate with Sin3a, a component of the NuRD histone deacetylase complex; 2) have decreased DNA‐binding affinity to the URE of Ikaros target genes, and 3) are unable to repress transcription of target genes in a luciferase reporter assay. The introduction of phosphoresistant Ikaros mutations restored the association of Ikaros with Sin3a, the ability Ikaros to bind DNA, and the transcriptional repression to the level observed in wild‐type Ikaros. These results show that PP1 regulates thymocyte differentiation by controlling Ikaros’ association with chromatin remodeling complexes and its ability to repress the transcription of developmentally regulated genes.