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© The Rockefeller University Press, 0021-9525/1998//1427 $5.00
The Journal of Cell Biology, Volume 143, Number 6, , 1998 1427-1436

Article

Chromosomal Proteins HMG-14 and HMG-17 Are Released from Mitotic Chromosomes and Imported into the Nucleus by Active Transport



Robert Hock*,{ddagger}, Ulrich Scheer{ddagger}, and Michael Bustin*

* Protein Section, Laboratory of Molecular Carcinogenesis, Division of Basic Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892; and {ddagger} Department of Cell and Developmental Biology, Biozentrum, University of Wurzburg, Am Hubland, D-97074, Wurzburg, Germany

The high mobility group 14/17 (HMG-14/ -17) proteins form specific complexes with nucleosome core particles and produce distinct footprints on nucleosomal DNA. Therefore, they could be an integral part of the chromatin fiber. Here we show that during the cell cycle these proteins are transiently dissociated from chromatin. They colocalize with the nuclear DNA in interphase and prophase but not in metaphase and anaphase. They relocate into the nucleus and colocalize again with the DNA in late telophase, concomitantly with the appearance of the nuclear envelope. Thus, these nucleosomal binding proteins are not always associated with chromatin. Using reconstituted nuclei and permeabilized cells, we demonstrate that these two small proteins, with a molecular mass <10 kD, are actively imported into the nucleus. We identify the major elements involved in the nuclear import of these chromosomal proteins: HMG-14/-17 proteins contain an intrinsic bipartite nuclear localization signal, and their entry into the nucleus through nuclear pores requires energy and the participation of importin {alpha}. These findings suggest that the cell cycle–related association of HMG-14/-17 with chromatin is dependent on, and perhaps regulated by, nuclear import processes.

Key Words: HMG proteins • nuclear transport • cell cycle • localization • nuclear proteins

Abbreviations used in this paper: APC, allophycocyanin; EB, egg buffer; HMG, high mobility group; 5-IAF, 5-iodoacetamido-fluorescein; NLS, nuclear localization signal; NPC, nuclear pore complexes.

This work was partially supported by grant HO1804/1-1 from the Deutsche Forschungsgemeinschaft to Robert Hock, and by an award from the Humboldt Foundation.


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