Progesterone-binding components of chick oviduct: analysis of receptor structure by limited proteolysis

An endogenous calcium-activated sulfhydryl protease in chick oviduct cytosol has been utilized to study the structure of the chick oviduct progesterone receptor subunits, progestophilins A (79 000 g/mol) and B (117 000 g/mol). The protease is not a normal component of the native progesterone receptor aggregate (6 and 8 S) complexes. Both receptor protein subunits (A and B) can be cleaved to two hormone-binding fragments, form IV (43 000 g/mol) and meroreceptor (23 000 g/mol). The meroreceptors obtained from the A and B proteins are indistinguishable from each other on the basis of both size (gel filtration chromatography) and charge (isoelectric focusing, pI 8.3). These findings suggest a structural similarity between the A and B proteins. The discovery of a weak deoxyribonucleic acid (DNA) binding activity for the B protein suggests an even greater similarity between B and A subunits, since the A subunit has previously b:en shown to bind to DNA. The proteolytic fragments do not bind to DNA-cellulose, implying that the hormone- and DNA-binding regions of the A and B proteins exist in separate domains.