A 6S progesterone receptor form has been identified in chick oviduct cytosol fractions. It has been shown to consist of equal amounts of two 4S receptor subunits, A and B, previously shown to bind in nuclei to DNA and chromatin sites respectively. The 6S form is tentatively defined as a dimer of both A and B subunits. It is present in cytosol under conditions in which large in vitro aggregate formation (8S) is prevented. The 6S dimer co-chromatographs with authentic receptor B on DEAE-cellulose, but does not bind to phosphocellulose. Dissociation of the dimer with salt or temperature liberates the receptor subunits in their monomeric 4S forms.
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Schrader et al. (1975) studied this question.