Oligomerization of pregnancy-specific beta 1-glycoprotein (SP1) at physiologic pH and ionic strength

Distribution of SP1, immunoreactivity among different plasma proteins: real molecular heterogeneity or adsorption of SP1-beta to other plasma proteins?

Three SP1-containing factors from pooled term pregnancy sera were subjected to crossed immunoelectrophoresis. New patterns as far as electrophoretic mobilities and shapes of the immunoprecipitates were revealed. The appearance of an additional anodic radioimmunoassayable activity in agarose electrophoresis of mixed SP1-alpha and SP1-beta suggested a binding capacity of SP1-alpha for SP1-beta determinants. In the serum of a single patient at the third trimester of pregnancy we also found two SP1 variants, possessing little radioimmunological reactivity and with crossed immunoelectrophoretic characteristics quite different from those of the 'usual' alpha and beta SP1 forms. These results suggest that, in this particular case, the overall SP1 production cannot be evaluated by competitive binding assay and, that in general, SP1 is a complex antigen the heterogeneity of which can be determined following adsorption of some beta epitopes to another serum protein.

Pregnancy-specific beta 1-glycoprotein: cDNA cloning, tissue expression, and species specificity of one member of the PS beta G family

Pregnancy-specific beta 1-glycoprotein (PS beta G) is a heterogeneous product of the human syncytiotrophoblast, closely related to the CEA-NCA multigene family. In the present study, immunoscreening was carried out with anti-PS beta G antibodies to isolate cDNA sequences from a placental lambda gt11 expression library. One 1847-bp cDNA clone comprising the major portion of the coding sequence of a putative 48-kd peptide was sequenced and characterized. Hybridization of human genomic DNA to the PS beta G sequence revealed a complex pattern of restriction fragments, a finding well in agreement with the assumption that there are several independent PS beta G genes. A variable PstI band was found in human DNA. Transfer blot analysis of human placental RNA identified transcript of 2.2kb and 1.7kb that appear transiently with increasing levels of expression during gestation. No hybridization of PS beta G cDNA to human RNA from liver, kidney, heart, thyroid, and ovary was observed. In analyses of placental RNA from mouse, goat, sheep, and cow, no corresponding transcripts could be detected, and DNA hybridization under low-stringency hybridization conditions resulted in very faint cross-reacting bands, presumably indicating sequences that were scarcely related. However, PS beta G-specific DNA sequencies with similar restriction patterns were found in primates. These results are compatible with the assumption of late evolutionary development of certain PS beta G sequences.

The human pregnancy-specific beta 1-glycoprotein (PS beta G) and the carcinoembryonic antigen (CEA)-related proteins are members of the same multigene family

Pregnancy-specific beta 1-glycoprotein (PS beta G), a major product of the placenta with unknown function, consists of a set of glycoproteins synthesized by the syncytiotrophoblast. We report here the molecular cloning of 3 cDNA encoding different members of the PS beta G family. Two clones (C, D) correspond to a single transcript undergoing differential splicing. The third one (E) originates from a different gene. All three clones have identical (C, D) or similar (E) coding sequences except for the last residues at their carboxyl end. They contain 93 residue motifs related to the ancestral Ig-like domain which makes them new members of this gene superfamily. A striking sequence similarity (50 to 60%) is observed between PS beta G and carcinoembryonic antigen (CEA)-related proteins. The evolutionary relationship between CEA and PS beta G points to a possible common function in the control of cell invasion and/or metastasis.

Processing of SP1 precursor in a cell-free system from poly(A+) mRNA of human placenta

Cell-free translation of polyadenylated mRNA from human term placenta in a wheat germ extract, after immunoprecipitation with antibodies directed against purified pregnant serum SP1, yielded a single polypeptide of 31 kDa. Addition of dog pancreatic microsomal vesicles to the translation system resulted in the appearance of two polypeptides, one of them of 46 kDa and the other of 28 kDa. Both polypeptides were protected from limited proteolysis and when the assay was performed with lytic detergent concentrations in addition to proteases, this protection was abolished indicating that the polypeptides were segregated into the microsomal vesicles. The cleavage of a signal peptide of 3 kDa from the 31 kDa primary translation product gives rise to 28 kDa and accounts for the slight increase in electrophoretic mobility. The treatment of the immunoprecipitated products with Endoglycosidase H and alpha-mannosidase, suggested that only the 46 kDa polypeptide is a glycoprotein. From the results obtained we conclude that SP1 is synthesized and processed to a glycoprotein of 46 kDa which would be a protomeric form of the oligomers reported in pregnant serum by other authors.

New placental proteins: chemistry, physiology and clinical use

In the past decade, several new placental proteins have been isolated and studied. The 'pregnancy-specific' beta 1-glycoprotein (SP1) is a major placental product with unusual physicochemical properties that has been extensively investigated, but its biological function remains uncertain. Pregnancy-associated plasma protein A (PAPP-A), a glycoprotein of mol. wt 400,000, has effects in vitro on the coagulation and complement cascades, probably by its properties of protease inhibition. Placental protein 5 (PP5) may be involved in the coagulation and fibrinolytic systems, and in follicle maturation and semen liquefaction. 'Placental protein 12' (PP12) is not a product of the placenta at all; it appears to be produced in the female genital tract under the influence of progesterone and may also be produced by proliferating liver cells. Further study may reveal new roles for these placental proteins beyond their traditional roles as tumour markers.