2 POPULATIONS OF THE ESTROGEN-RECEPTOR SEPARATED AND CHARACTERIZED USING AQUEOUS 2-PHASE PARTITIONING

TWO populations of the rat uterine estrogen receptor (ER) were separat ed and characterized using aqueous two-phase partitioning. Countercurr ent distribution of rat uterine cytosolic ER allowed rapid and efficie nt separation of two populations, one population partitioned preferent ially into the top phase (T, K-obs = 3-6) and the other into the botto m phase (B, K-obs = 0.01-0.03) The majority of unoccupied cytosolic ER is in the T population. Upon estrogen binding and/or heating to 30 de grees C in vitro the T population is converted to the B population. Th e transition from T to B does not exclusively involve loss of heat sho ck protein 90 and does not alter the ligand binding ability of the ste roid binding domain. Using the human ER steroid binding domain overpro duced in Escherichia coli and the steroid binding domain generated by partial trypsinization of the rat uterine ER, we demonstrate that the characteristic distinguishing T and B populations is not localized to this domain alone but may be associated with the amino terminal half o f the ER (the A/B and DNA binding domains). The T to B transition of t he ER also occurs in human MCF-7 breast cancer cells upon treatment wi th estrogen at 37 degrees C.