Characterization of cDNA encoding human pregnancy-specific beta 1-glycoprotein from placenta and extraplacental tissues and their comparison with carcinoembryonic antigen.

Human pregnancy-specific beta 1-glycoprotein (SP1) is one of the early pregnancy proteins produced in large quantity by the placenta during pregnancy. The nucleotide sequence of a cDNA encoding human placental SP1 isolated from a term placental cDNA library was determined. This cDNA contains 1,958 nucleotides with a 5' noncoding sequence of 73 bp, a sequence of 1,257 bp encoding a protein of 419 amino acids with calculated molecular mass of 47.2 kD, a TGA stop codon, and 625 bp of 3' noncoding sequence. Two internal repeat domains, each of 93 amino acids, can be identified within the coding sequence. The positions of two cysteine residues within each repeat are conserved. A cDNA of 489 bp identical to the sequence from nucleotides 422-910 of placental SP1 cDNA was isolated from a human testis cDNA library. Screening of a HeLa cell library with SP1 cDNA probe yielded 10 positive clones. Sequence determination of one of the cloned cDNA inserts revealed a partial cDNA of 773 bp which is 94% homologous at the nucleotide level and 88% homologous at the amino acid level to the placental SP1 cDNA. These observations confirm the observation that some of the SP1 genes might be expressed in extraplacental tissues. Searching through the GenBank/EMBL database revealed 74-80% homology between SP1 cDNA with human carcinoembryonic antigen (CEA) cDNA. The multiple genes of SP1 as a subfamily of the immunoglobulin supergene family is implicated.