We have previously shown, in the
estrogen-unresponsive C3H mouse mammary
tumor that the affinity of the
estrogen receptor (ER) for
calf thymus DNA in vitro is four-times higher than that of uterine ER [Baskevitch, P. P., Vignon, F., Bousquet, C. and Rochefort, H. (1983)
Cancer Res. 43, 2290]. By mixing cytosols from this
tumor and uterus, we describe a
tumor factor capable of increasing ER affinity for
DNA, as assayed by
DNA-cellulose chromatography and saturation studies. The activity of this factor was inhibited by
alpha-chymotrypsin-inhibitors such as N-tosylphenylalanylchloromethane and
chymostatin. Using the
fluorogenic substrate glutarylglycylglycylphenylalanyl-N-naphthylamide, we assayed such a
protease in the C3H mammary
tumor cytosol. This
protease and the factor altering ER-
DNA binding were eluted together from chromatography on
DEAE-cellulose, AcA 44, and
carboline-
agarose and were sensitive to the same inhibitors. The partially purified factor decreases the molecular mass of the
estrogen receptor as seen by denaturing electrophoresis after covalent labelling of the ER with [3H]
tamoxifen aziridine. We suggest that the increase of ER affinity for
DNA and the decrease of ER molecular size are due to the same
protease with an
alpha-chymotrypsin-like specificity.