Molecular cloning of complementary DNAs for two human endometrial proteins and cellular localization of their...

Insulin-like growth factor binding protein-1 (IGFBP-1) and a human beta-lactoglobulin homologue (beta LG/PP14) are two major secretory proteins of the human endometrium. The genes coding for these two proteins are expressed in separate types of the endometrial cells, with the IGFBP-1 gene being expressed in the stromal and the beta LG/PP14 gene in the glandular epithelial cells. Although the biological reasons for the presence and expression of IGFBP-1 and beta LG/PP14 in human endometrial cells remain to be elucidated, the fact that these gene products are expressed in different endometrial cell types provides a unique opportunity to employ them as markers in studies on epithelial-to-stromal cell communication in the endometrium. The primary structures of these proteins have been deduced from their cloned cDNAs. beta LG/PP14 is highly homologous to all known beta-lactoglobulins from various species. For example, horse beta-lactoglobulin I monomer exhibits a 53% protein sequence identity with beta LG/PP14; they have the same number of amino acid residues, and their three-dimensional structures are predicted to be similar. This latter conclusion is inferred from the fact that the four cysteinyl residues that are responsible for the formation of intramolecular bridges in beta-lactoglobulins are spatially conserved in beta LG/PP14. The human protein is encoded by a 900-base pair-long mRNA that is expressed in the glandular epithelial cells of the endometrium in a cyclic manner; in addition, it is found in the mucosal epithelial cells of the fallopian tubes. Several lines of circumstantial evidence suggest that the expression of the beta LG/PP14 gene is regulated by progesterone; however, whether this regulation is elicited by the progesterone receptor at the transcriptional level has not so far been demonstrated. The IGFBP-1 protein sequence contains 259 amino acid residues, with the propeptide possessing a 25-amino acid-long signal peptide. The NH2-terminal sequence of this and other IGFBPs is very cysteine-rich, suggesting the possibility that this domain is involved in the binding of IGF-I and IGF-II ligands. A PEST region, a sequence that is found in proteins with short intracellular half-lives, is included in the middle half of the IGFBP-1 polypeptide. Among the IGFBPs, IGFBP-1 appears to be the only one with a PEST sequence. The carboxy-terminal end of IGFBP-1 contains an Arg-Gly-Asp tripeptide that is also found in IGFBP-2 and may function as a cell attachment recognition signal in these proteins.(ABSTRACT TRUNCATED AT 400 WORDS)