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Plant Cell Advance Online Publication
Published on November 17, 2006; 10.1105/tpc.106.047274

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Received September 8, 2006
Returned for revision October 2, 2006
Accepted October 30, 2006

Arabidopsis Chromatin-Associated HMGA and HMGB Use Different Nuclear Targeting Signals and Display Highly Dynamic Localization within the Nucleus

Dorte Launholt 1, Thomas Merkle 2, Andreas Houben 3, Alexander Schulz 4, and Klaus D. Grasser 1*

1 Department of Life Sciences, Aalborg University, DK-9000 Aalborg, Denmark
2 Department of Genome Research, University of Bielefeld, D-33594 Bielefeld, Germany
3 Leibniz Institute of Plant Genetics and Crop Plant Research, D-06466 Gatersleben, Germany
4 Department of Plant Biology, Royal Veterinary and Agricultural University, DK-1871 Frederiksberg, Denmark

* To whom correspondence should be addressed. E-mail: kdg{at}bio.aau.dk.

In plants, the chromatin-associated high mobility group (HMG) proteins occur in two subfamilies termed HMGA and HMGB. The HMGA proteins are characterized by the presence of four AT-hook DNA binding motifs, and the HMGB proteins contain an HMG box DNA binding domain. As architectural factors, the HMG proteins appear to be involved in the regulation of transcription and other DNA-dependent processes. We have examined the subcellular localization of Arabidopsis thaliana HMGA, HMGB1, and HMGB5, revealing that they localize to the cell nucleus. They display a speckled distribution pattern throughout the chromatin of interphase nuclei, whereas none of the proteins associate with condensed mitotic chromosomes. HMGA is targeted to the nucleus by a monopartite nuclear localization signal, while efficient nuclear accumulation of HMGB1/5 requires large portions of the basic N-terminal part of the proteins. The acidic C-terminal domain interferes with nucleolar targeting of HMGB1. Fluorescence recovery after photobleaching experiments revealed that HMGA and HMGB proteins are extremely dynamic in the nucleus, indicating that they bind chromatin only transiently before moving on to the next site, thereby continuously scanning the genome for targets. By contrast, the majority of histone H2B is basically immobile within the nucleus, while linker histone H1.2 is relatively mobile.






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