MAMMARY-DERIVED GROWTH INHIBITOR IS NOT A DISTINCT PROTEIN BUT A MIX OF HEART-TYPE AND ADIPOCYTE-TYPE FATTY-ACID-BINDING PROTEIN

The amino acid sequence of the mammary derived growth inhibitor (MDGI) from bovine mammary gland (Bohmer, F.-D., Kraft, R., Otto, A., Wernst edt, C., Hell man, U., Kurtz, A., Muller, T., Rohde, K., Etzold, G., L ehmann, W., Langen, P., Heldin, C.-H., and Grosse, R. (1987) J. Biol. Chem. 262, 15137-15143) revealed 95% identity to bovine heart fatty ac id-binding protein (H-FABP), explaining the observed immunocross-react ivity. However, a cDNA encoding MDGI has not been found to date. Artif icial MDGI cDNA was expressed in an in vitro transcription/translation assay. Analysis by isoelectric focusing of the immunoprecipitated in vitro translation products of lactating bovine mammary gland mRNA did not indicate a protein corresponding to the in vitro translation produ ct of artificial MDGI mRNA. Moreover, two-dimensional electrophoresis of bovine mammary gland proteins confirmed the absence of a protein wi th the pi of the in vitro translated artificial MDGI mRNA in bovine ma mmary gland and instead revealed, apart from H-FABP, an unknown protei n that was recognized by anti-H-FABP antibodies. From lactating bovine mammary gland the cDNA for adipocyte fatty acid-binding protein (A-FA BP) was cloned. The in vitro translation of recombinant mRNA derived f rom this cDNA yielded a polypeptide that behaved like the unknown immu noreactive protein. Western blotting and immunofluorescence using mono specific antibodies demonstrated the coexistence of H-FABP and A-FABP in the lactating mammary gland. Taking into account that deviations of the MDGI sequence from the bovine H-FABP sequence correspond with A-F ABP we attribute the structure originally reported as MDGI to a mix of these proteins.