Purification and characterization of β-adrenergic receptor mRNA-binding proteins

beta-Adrenergic receptors (beta-ARs), like other G-protein-coupled receptors, can undergo post-transciptional regulation at the level of mRNA stability. In particular, the human beta(1)- and beta(2)-ARs and the hamster beta(2)-AR mRNA undergo beta-agonist-mediated destabilization. By UV cross-linking, we have previously described an similar to M-r 36,000 mRNA-binding protein, beta ARB, that binds to A/C+U-rich nucleotide regions within 3'-untranslated regions. Further, we have demonstrated previously that beta ARB is immunologically distinct from AUF1/heterogeneous nuclear ribonucleoprotein (hnRNP) D, another mRNA-binding protein associated with destabilization of A+U-rich mRNAs (Pende, A., Tremmel, K. D., De-Maria, C. T., Blaxall, B. C., Minobe, W., Sherman, J. A., Bisognano, J., Bristow, M. R., Brewer, G., and Port, J. D. (1996) J. Biol. Chem. 271, 8493-8501). In this report, we describe the peptide composition of beta ARB. Mass spectrometric analysis of an similar to M-r 36,000 band isolated from ribosomal salt wash proteins revealed the presence of two mRNA-binding proteins, hnRNP Al, and the elav-like protein, HuR, both of which are known to bind to A+U-rich nucleotide regions. By immunoprecipitation, HuR appears to be the biologically dominant RNA binding component of beta ARB. Although hnRNP Al and HuR can both be immunoprecipitated from ribosomal salt wash proteins, the composition of beta ARB (HuR alone versus HuR and hnRNP Al) appears to be dependent on the mRNA probe used. The exact role of HuR and hnRNP Al in the regulation of beta-AR mRNA stability remains to be determined.