Isolation and characterization of complementary DNAs encoding human pregnancy-specific beta 1-glycoprotein

Pregnancy-specific beta 1-glycoprotein (PS beta G) isolated from human placenta consists of a set of at least three glycoproteins with apparent molecular masses of 72, 64, and 54 kDa, respectively. This heterogeneity is confirmed by the detection of three nonglycosylated polypeptides of 50, 48, and 36 kDa, which can be immunoprecipitated by antiserum to placental PS beta G obtained by in vitro translation of placental poly(A)+ RNA. To examine the structural relationships between these proteins, two cDNA clones of 1912 base pairs (PSG16) and 2131 base pairs (PSG93) encoding human PS beta Gs were isolated from a human placental lambda gt11 cDNA library. The sequenced portions of these two cDNAs are identical with the exception that clone PSG93 contains an additional 86 base pairs at the end of the common 3'-coding region. This insertion could result in the generation of a PS beta G species of 419 amino acid residues instead of the 417 amino acid residues predicted by the sequence of clone PSG16. The calculated molecular masses of the two polypeptides encoded by PSG16 and PSG93 are 46.9 and 47.2 kDa, close to the size of the major nonglycosylated PS beta G of 48 kDa. The identity of proteins coded for by these cDNA clones was confirmed by comparing the predicted amino acid sequences to sequences determined from endoproteinase Lys-C peptides obtained from human placental PS beta G. Two placental PS beta G mRNAs of 2200 bases (major) and 1700 bases (minor) have been detected by Northern hybridization analysis. Primer extension and S1 nuclease mapping experiments demonstrated that PS beta G mRNAs have heterogeneous 5' termini.