HIGH-MOBILITY GROUP-1 PROTEIN IS NOT STABLY ASSOCIATED WITH THE CHROMOSOMES OF SOMATIC-CELLS

High mobility group 1 (HMG1) protein is an abundant and conserved comp onent of vertebrate nuclei and has been proposed to play a structural role in chromatin organization, possibly similar to that of histone H1 . However, a high abundance of HMG1 had also been reported in the cyto plasm and on the surface of mammalian cells. We conclusively show that HMG1 is a nuclear protein, since several different anti-HMG1 antibodi es stain the nucleoplasm of cultured cells, and epitope-tagged HMG1 is localized in the nucleus only. The protein is excluded from nucleoli and is not associated to specific nuclear structures but rather appear s to be uniformly distributed. HMG1 can bind in vitro to reconstituted core nucleosomes but is not stably associated to chromatin in live ce lls. At metaphase, HMG1 is detached from condensed chromosomes, contra ry to histone H1. During interphase, HMG1 readily diffuses out of nucl ei after permeabilization of the nuclear membranes with detergents, wh ereas histone H1 remains associated to chromatin. These properties exc lude a shared function for HMG1 and H1 in differentiated cells, in spi te of their similar biochemical properties. HMG1 may be stably associa ted only to a very minor population of nucleosomes or may interact tra nsiently with nucleosomes during dynamic processes of chromatin remode ling.