A MAJOR ESTROGEN-REGULATED PROTEIN SECRETED FROM THE LIVER OF XENOPUS-LAEVIS IS A MEMBER OF THE SERPIN SUPERFAMILY - NUCLEOTIDE-SEQUENCE OFCDNA AND HORMONAL INDUCTION OF MESSENGER-RNA

Estrogen treatment of Xenopus frogs causes four mRNAs to become highly abundant in the liver. Three of these mRNAs have been previously iden tified as coding for vitellogenin, ferritin, and serum retinol binding protein. We show here that the fourth abundant liver messenger RNA co mprises about 1500 nucleotides and codes for a 45-kDa secreted protein , designated Ep45. A clone complementary to Ep45 mRNA was isolated, an d its identity was confirmed by hybridization selection of mRNA that t ranslated in vitro into the Ep45 precursor. Nucleotide sequence analys is of the nearly full length cDNA revealed a total length of 1454 base pairs consisting of: 36 nucleotides of the 5' noncoding region, 1308 base pairs encoding an open reading frame of 436 amino acids, and 110 nucleotides of the 3' untranslated region. Ep45 mRNA may originate fro m as many as four closely spaced transcription start sites, which are 15 to 21 bases upstream of the first nucleotide of the cDNA clone. The Xenopus laevis genome appears to contain a single Ep45 gene. The dedu ced amino acid sequence indicates that Ep45 has features typical of a secreted protein, including a signal peptide of 16 amino acids and thr ee potential sites for N-linked glycosylation, and is related to the s erine protease inhibitors, a large family of proteins with very divers e physiological functions. Ep45 mRNA was absent in the liver of normal male frogs and increased at least 100-fold in response to estradiol-1 7-beta. Thus, both Ep45 and vitellogenin mRNAs are switched from undet ectable to very high levels, a pattern of expression not found for any other mRNAs in Xenopus liver.