Sex steroid-binding protein: identification and comparison of the primary product following cell-free translation of human and monkey (Macaca fascicularis) liver RNA

A very close similarity in molecular, steroid-binding and immunological properties have been demonstrated for the sex steroid-binding proteins of plasma from human (hSBP) and monkey (mSBP): both are glycoproteins composed of two similar subunits able to bind one steroid molecule and to cross-react with the same antibodies. After translation of human and monkey (Macaca fascicularis) liver mRNAs by a wheat-germ embryo extract, in the presence of labelled amino-acids, we have characterized in both cases a single radioactive polypeptide immunologically related to SBP, migrating in SDS-PAGE as a single band and having a molecular weight of about 42,000. This protein could be displaced from the antibody by pure unlabelled SBP in excess. The difference in molecular weight between the in vitro translation product and the native SBP sub-unit is probably due to the absence of glycosylation in the neo-synthesized protein. The radioactivity incorporated into mSBP was 4 times higher than the radioactivity incorporated into hSBP, suggesting that the amount of mRNA for SBP is higher in monkey than in human liver. Our results show that the two sub-units of hSBP and mSBP derive from a common precursor, representing respectively 0.0050% and 0.0013% of the total neosynthesized proteins in monkey and in human liver.