Isolation and characterization of a folate receptor mRNA-binding trans-factor from human placenta - Evidence favoring identity with heterogeneous nuclear ribonucleoprotein E1

The interaction of an 18-base cis-element in the 5'-untranslated region of human folate receptor (FR)-alpha mRNA with a cytosolic trans-factor protein is critical for the translation of FR (Sun, X.-L., and Antony, A. C. (1996 ) J. Biol. Chem. 271, 25539-25547). This trans-factor was isolated to appar ent homogeneity as a 43- and 38-kDa doublet from human placenta using poly( U)-Sepharose, followed by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electro-elution as major purification steps. Amino acid microsequencing of two cyanogen bromide-generated peptide fragments o f the 43-kDa trans-factor revealed complete identity with 43-kDa heterogene ous nuclear ribonucleoprotein E1 (hnRNP E1). Purified specific rabbit anti- hnRNP E1 peptide antibodies (generated against a synthetic oligopeptide tha t was not represented in microsequenced peptides of the trans-factor) also recognized the purified trans-factor on Western blots. Conversely, the 18-b ase FR RNA cis-element also bound hnRNP E1 protein on Northwestern blots. M oreover, a 19-base RNA cis-element in the 3'-untranslated region of 15-lipo xygenase mRNA that is known to bind hnRNP E1 also interacted with placental 43-kDa trans-factor. In addition, several murine tissues containing a hnRN P E1-related protein (also known as alpha CP-1) readily interacted with the 18-base FR RNA cis-element. Finally, anti-hnRNP El antibodies specifically inhibited translation of FR in vitro in a dose-dependent manner, and the a ntibody effect could be reversed in a dose-dependent manner by either purif ied trans-factor or hnRNP E1. Collectively, the data favor identity of the FR mRNA-binding trans-factor and hnRNP E1, confirm its critical role in the translation of FR, and highlight yet another role of multifunctional hnRNP E1 in eukaryotic mRNA regulation.