Ikaros Stability and Pericentromeric Localization Are Regulated by Protein Phosphatase 1

Abstract

Ikaros encodes a zinc finger protein that is involved in gene regulation and chromatin remodeling. The majority of Ikaros localizes at pericentromeric heterochromatin (PC-HC) where it regulates expression of target genes. Ikaros function is controlled by posttranslational modification. Phosphorylation of Ikaros by CK2 kinase determines its ability to bind DNA and exert cell cycle control as well as its subcellular localization. We report that Ikaros interacts with protein phosphatase 1 (PP1) via a conserved PP1 binding motif, RVXF, in the C-terminal end of the Ikaros protein. Point mutations of the RVXF motif abolish Ikaros-PP1 interaction and result in decreased DNA binding, an inability to localize to PC-HC, and rapid degradation of the Ikaros protein. The introduction of alanine mutations at CK2-phosphorylated residues increases the half-life of the PP1-nonbinding Ikaros mutant. This suggests that dephosphorylation of these sites by PP1 stabilizes the Ikaros protein and prevents its degradation. In the nucleus, Ikaros forms complexes with ubiquitin, providing evidence that Ikaros degradation involves the ubiquitin/proteasome pathway. In vivo, Ikaros can target PP1 to the nucleus, and a fraction of PP1 colocalizes with Ikaros at PC-HC. These data suggest a novel function for the Ikaros protein; that is, the targeting of PP1 to PC-HC and other chromatin structures. We propose a model whereby the function of Ikaros is controlled by the CK2 and PP1 pathways and that a balance between these two signal transduction pathways is essential for normal cellular function and for the prevention of malignant transformation.

The making of a lymphocyte: the choice among disparate cell fates and the IKAROS enigma

“…Phosphorylation of this region by casein kinase II (CKII) occurs during the G1-S transition and is responsible for reducing the DNA-binding activity of IKAROS proteins (Gomez-del ). This phosphorylation event may also promote protein degradation through an associated PEST motif and can be negatively regulated by protein phosphatase 1 (PP1) (Popescu et al 2009). Additional IKAROS phosphorylation events that involve the serine and threonine residues at the N-terminal zinc finger linker regions occur at the M phase of the cell cycle and also interfere with DNA binding ( Fig.…”

Transcription factor Ikaros Represses Protein Phosphatase 2A (PP2A) Expression through an Intronic Binding Site

“…(14). One of the proteins with which it is known to interact is histone deacetylase 1 or For real-time PCR, the housekeeping gene ␤-actin was used for normalization.…”

Conserved Residues in the N Terminus of Lipin-1 Are Required for Binding to Protein Phosphatase-1c, Nuclear Translocation, and Phosphatidate Phosphatase Activity

“…This could be expected if lipin-bound PP-1c only contributes a small proportion of the nuclear PP-1c. However, other nuclear-localized PP-1c regulatory proteins, such as Ikaros, do promote nuclear localization of PP-1c when overexpressed (44). Perhaps PP-1c could facilitate lipin-1 nuclear entry but is not itself imported into the nucleus with lipin-1.…”