title: Human Ikaros function in activated T cells is regulated by coordinated expression of its largest isoforms.
abstract: The Ikaros gene is alternately spliced to generate multiple zinc finger proteins involved in gene regulation and chromatin remodeling. Whereas murine studies have provided important information regarding the role of Ikaros in the mouse, little is known of Ikaros function in human. We report functional analyses of the two largest human Ikaros (hIK) isoforms, hIK-VI and hIK-H, in T cells. Abundant expression of hIK-H, the largest described isoform, is restricted to human hematopoietic cells. We find that the DNA binding affinity of hIK-H differs from that of hIK-VI. Co-expression of hIk-H with hIk-VI alters the ability of Ikaros complexes to bind DNA motifs found in pericentromeric heterochromatin (PC-HC). In the nucleus, hIK-VI is localized solely in PC-HC, whereas the hIK-H protein exhibits dual centromeric and non-centromeric localization. Mutational analysis defined the amino acids responsible for the distinct DNA binding ability of hIK-H, as well as the sequence required for the specific subcellular localization of this isoform. In proliferating cells, the binding of hIK-H to the upstream regulatory region of known Ikaros target genes correlates with their positive regulation by Ikaros. Results suggest that expression of hIK-H protein restricts affinity of Ikaros protein complexes toward specific PC-HC repeats. We propose a model, whereby the binding of hIK-H-deficient Ikaros complexes to the regulatory sequence of target genes would recruit these genes to the restrictive pericentromeric compartment, resulting in their repression. The presence of hIK-H in the Ikaros complex would alter its affinity for PC-HC, leading to chromatin remodeling and activation of target genes.
authors.affiliation: Mattel Children's Hospital, Los Angeles, California 90095, USA.
authors.forename: Tapani
authors.initials: T
authors.lastname: Ronni
authors.forename: Kimberly J
authors.initials: KJ
authors.lastname: Payne
authors.forename: Sam
authors.initials: S
authors.lastname: Ho
authors.forename: Michelle N
authors.initials: MN
authors.lastname: Bradley
authors.forename: Glenn
authors.initials: G
authors.lastname: Dorsam
authors.forename: Sinisa
authors.initials: S
authors.lastname: Dovat
mesh_terms: D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D006801:Humans; D051740:Ikaros Transcription Factor / Q000235:genetics / Q000378:metabolism; D008213:Lymphocyte Activation / Q000235:genetics; D051379:Mice; D008958:Models, Molecular; D011485:Protein Binding; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D013601:T-Lymphocytes / Q000378:metabolism*
chemical_list: D004268:DNA-Binding Proteins; C497239:IKZF1 protein, human; D020033:Protein Isoforms; D051740:Ikaros Transcription Factor