THE ESTROGEN-INDUCIBLE TRANSFERRIN RECEPTOR-LIKE MEMBRANE GLYCOPROTEIN IS RELATED TO STRESS-RELATED PROTEINS

It was previously shown that estrogen induces a membrane glycoprotein (molecular mass, 95 kDa) in the chicken oviducts, which exhibits sever al properties similar to transferrin receptors (Poola, I., and Lucas, J. J. (1988) J. Biol. Chem. 263, 19137-19146). In the present study, w e have further investigated its molecular and transferrin binding prop erties. We have sequenced several internal peptides isolated from the purified protein by endopeptidase Lys-C. We have found that it has a h igh degree of sequence homologies with those of chicken heat-shock pro tein (cHsp108), mouse endoplasmic reticulum protein (mERp99), hamster glucose-regulated protein (hagrp94), and human tumor rejection antigen hTRAgp96), all of which are shown to be highly homologous to each oth er and to yeast hsp90. We demonstrate here that the [S-35]methionine-l abeled immunoaffinity-purified estrogen-inducible membrane glycoprotei n binds to the transferrin affinity columns similar to iron-modulated transferrin receptors. Indirect immunofluorescence microscopic studies indicate that it is an intracellular glycoprotein unlike transferrin receptors. We have isolated two molecular forms of the protein, with m olecular masses of 116 and 104 kDa, by immunoaffinity column purificat ion, immunoprecipitation, Western blotting, and pulse-chase labeling a nalyses. Both 116- and 104-kDa species bind transferrin. This protein can be induced by heat shocking the oviduct cells at 45 degrees C for 3 h and recovering at 37 degrees C for 2-3 h. It is also expressed in the human breast cancer cell lines, MCF-7 and T-47D. All these propert ies taken together strongly suggest that the estrogen-inducible membra ne glycoprotein is a novel transferrin-binding protein, structurally r elated to the stress-regulated proteins.