We have studied JMJD2b histone demethylase, which antagonizes H3K9me3 in the pericentromeric heterochromatin. In cells with a deficiency in the histone methyltransferase SUV39h, the level of full-length JMJD2b (JMJD2b-GFP-1086) at chromocenters was reduced, corresponding to a global decrease in JMJD2b and H3K9me3. In wild-type fibroblasts, the chromatin of ribosomal genes, which is dense with H3K9 methylation, lacked JMJD2b-GFP-1086, while mutant and truncated forms of JMJD2b densely occupied the nucleolar compartment. This implies that the PHD Zn-fingers and Tudor domains, which were removed in truncated JMJD2b, are responsible for the aberrant JMJD2b function. Intriguingly, the JMJD2b-GFP-1086 level was significantly higher in tumor cell nucleoli. The kinetic properties of JMJD2b-GFP-1086 in the nucleoli and nucleoplasm of normal and tumor cells were similar; ∼50% recovery of prebleached intensity was reached after <1 s. However, the mobile fraction of JMJD2b-GFP-1086 was increased in SUV39h-deficient cells. Similarly, the mobile fractions of mutant JMJD2b(1-424)H189A-GFP and truncated JMJD2b(1-424)GFP were greater than that measured for the full-length protein. We suggest that nucleoli are the site of an aberrant function of JMJD2b, the kinetic properties of which can be influenced by a mutant genetic background.

Institute of Biophysics Academy of Sciences of the Czech Republic v v i Královopolská 135 CZ 612 65 Brno Czech Republic

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