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* Dipartimento di Genetica e di Biologia dei Microrganismi, Universitá di Milano, 20133 Milano, Italy; High mobility group 1 (HMG1) protein is an
abundant and conserved component 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 cytoplasm and on the surface
of mammalian cells. We conclusively show that HMG1
is a nuclear protein, since several different anti-HMG1
antibodies 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 appears to
be uniformly distributed. HMG1 can bind in vitro to reconstituted core nucleosomes but is not stably associated to chromatin in live cells. At metaphase, HMG1 is
detached from condensed chromosomes, contrary to
histone H1. During interphase, HMG1 readily diffuses
out of nuclei after permeabilization of the nuclear
membranes with detergents, whereas histone H1 remains associated to chromatin. These properties exclude a shared function for HMG1 and H1 in differentiated cells, in spite of their similar biochemical
properties. HMG1 may be stably associated only to a
very minor population of nucleosomes or may interact
transiently with nucleosomes during dynamic processes of chromatin remodeling.
DIBIT, San Raffaele
Scientific Institute, 20132 Milano, Italy; and § Deutsches Krebsforschungszentrum, 69120 Heidelberg, Germany
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