Heterologous expression and characterization of prolactin-like protein-A. Identification of serum binding proteins

In this report, we describe the heterologous expression of prolactin-like protein-A (PLP-A) in Chinese hamster ovary (CHO) cells, the characterization of recombinant PLP-A, and the identification of serum PLP-A-binding proteins. CHO cell and native placental PLP-A showed similar immunoreactive characteristics and electrophoretic mobilities. N-terminal sequencing verified the identity and purity of the recombinant PLP-A species and the site of cleavage of the signal peptide from the mature secreted PLP-A species. Recombinant PLP-A lacked activity in standardized prolactin and growth hormone in vitro bioassays. Antibodies generated to recombinant PLP-A facilitated the cellular localization of PLP-A and the identification of high molecular weight PLP-A complexes. Cross-linking analyses of radioiodinated PLP-A with serum harvested from late gestation rats indicated the presence of two major cross-linked complexes migrating under reducing conditions at 130 and 250 kDa and two minor cross-linked complexes migrating at 70 and 110 kDa. Binding of PLP-A to serum proteins was specific for PLP-A and not effectively competed by other members of the prolactin/growth hormone family. The PLP-A binding species were also found in serum from non-pregnant female and male rats.