JBC, Vol. 250, Issue 8, 2904-2910, Apr, 1975
Glucocorticoid receptors in lung. Comparison between nonactivated and activated forms of the cytoplasmic glucocorticoid binding protein and their relationship to the nuclear binding protein of fetal rabbit lung
G. Giannopoulos
In the absence of salt the cytoplasmic glucocorticoid receptor of fetal
rabbit lung sediments at 7 S while the nuclear receptor sediments at 4 S.
However, if nuclear extracts are mixed with receptor-depleted cytosol
preparations in dilute buffer solutions without added salt, the nuclear 4 S
receptor sediments as a 7 S species similar to that observed for the
cytoplasmic form under the same conditions suggesting an interaction of the
nuclear receptor with other cytosol proteins rather than with itself. In
addition, both cytoplasmic and nuclear receptors sediment at 4 S in 0.4 M
KCl and a major fraction of the nuclear receptor has an agarose elution
profile identical to that of the cytoplasmic receptor. Thus a major
fraction of the nuclear receptors is indistinguishable from the cytoplasmic
receptors by the methods used. Since the cytoplasmic receptor sediments at
4 S in 0.15 M KCl, it is suggested that in vivo the glucocorticoid receptor
may exist as a 4 S species and that the 7 S form described previously may
result from an interaction of the 4 S component with other cytosol proteins
in hypotonic media. About 25% of the receptor present in nuclear extracts
has an agarose elution profile different from that of the cytoplasmic
receptor in 0.4 M KCl. This suggests that either the nuclear receptor
associates with itself or other nuclear proteins or that more than one form
of nuclear receptor exists. Earlier observations suggested that in the
absence of hormone the glucocorticoid receptor is localized exclusively in
the cytoplasm of lung cells and that the nuclear receptor is formed by a
transfer of the cytoplasmic steroid-receptor complex into the nucleus. A
prerequisite for this transfer seems to be a modification of the receptor
to an active form which can bind to nuclei. This receptor transfomration,
referred to in this paper as activation of the receptor, can occur in the
absence of nuclei and is highly dependent on temperature and ionic
strength. Cytoplasmic receptors activated either by heating or by exposure
to high ionic strength are indistinguishable from nonactivated receptors by
sucrose density gradient analysis or by agarose gel filtration in solutions
containing 0.4 M KCl. Simiarly, no significant difference in the absence of
salt is observed after activation by heating. These results suggest that
activation of the cytoplasmic glucocorticoid receptor involves
conformational changes which favor its transfer and/or binding to nuclear
sites rather than conversion of a 4 S species to a faster-sedimenting form
by dimerization or by addition of another protein unit as has been proposed
for the activation of the estrogen receptor of the rat uterus.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
