The plant-specific family of DNA-binding proteins containing three HMG-box domains interacts with mitotic and meiotic chromosomes

The high mobility group (HMG)-box represents a DNA-binding domain that is found in various eukaryotic DNA-interacting proteins. Proteins that contain three copies of the HMG-box domain, termed 3 x HMG-box proteins, appear to be specific to plants. The Arabidopsis genome encodes two 3 x HMG-box proteins that were studied here. DNA interactions were examined using electrophoretic mobility shift assays, whereas expression, subcellular localization and chromosome association were mainly analysed by different types of fluorescence microscopy. The 3 x HMG-box proteins bind structure specifically to DNA, display DNA bending activity and, in addition to the three HMG-box domains, the basic N-terminal domain contributes to DNA binding. The expression of the two Arabidopsis genes encoding 3 x HMG-box proteins is linked to cell proliferation. In synchronized cells, expression is cell cycle dependent and peaks in cells undergoing mitosis. 3 x HMG-box proteins are excluded from the nuclei of interphase cells and localize to the cytosol, but, during mitosis, they associate with condensed chromosomes. The 3 x HMG-box2 protein generally associates with mitotic chromosomes, while 3 x HMG-box1 is detected specifically at 45S rDNA loci. In addition to mitotic chromosomes the 3 x HMG-box proteins associate with meiotic chromosomes, suggesting that they are involved in a general process of chromosome function related to cell division, such as chromosome condensation and/or segregation.