Isoforms of fatty-acid-binding protein in bovine heart are coded by distinct mRNA

In the course of our studies on structure/function relationships of fatty-acid-binding proteins, we reported earlier that the two isoforms of the 15-kDa cardiac fatty-acid-binding protein (cFABP) from bovine heart only differ in one position; Asn98 in pI 5.1-cFABP; Asp98 in pI 4.9-cFABP [Unterberg, C., Börchers, T., Højrup, P., Knudsen, J. and Spener, F. (1990) J. Biol. Chem. 265, 16,255-16,261]. In the present study, we elucidate the origin for this heterogeneity. Isoelectric focusing analysis of immunoprecipitated in vitro translation products from total mRNA and positive-hybrid-selected cFABP/mRNA revealed two L-[35S]methionine-labeled proteins corresponding to pI 5.1-cFABP and pI 4.9-cFABP. In a control experiment, recombinant mRNA derived from cDNA encoding pI 5.1-cFABP was translated and produced only pI 5.1-cFABP as shown by isoelectric focusing of the translation products. We could observe co-translational acetylation but not post-translational deamidation of the cFABP isoforms. Taken together, our results demonstrate that the isoforms of cardiac fatty-acid-binding protein found in bovine heart are coded by distinct mRNA species.