BETA-ADRENERGIC AGONISTS THAT DOWN-REGULATE RECEPTOR MESSENGER-RNA UP-REGULATE A M(R) 35,000 PROTEIN(S) THAT SELECTIVELY BINDS TO BETA-ADRENERGIC-RECEPTOR MESSENGER-RNAS

In an effort to explore the molecular basis for agonist-induced destabilization of beta-adrenergic receptor mRNA, we investigated the nature of RNA-binding proteins both in untreated and agonist-treated DDT1-MF2 smooth muscle cells. Messenger RNAs for the alpha1b-, beta1, and beta2-adrenergic receptors as well as for beta-globin were transcribed in vitro, incubated with cytosolic fractions, covalently cross-linked by short-wave UV light, and analyzed by SDS-polyacrylamide gel electrophoresis. A prominent M(r) 35,000 radiolabeled protein(s) with the following characteristics was identified: (i) binds selectively to beta1- and beta2-adrenergic receptor mRNAs, both of which undergo agonist-induced down-regulation; (ii) does not bind to either alpha1b-adrenergic receptor mRNA, which does not undergo agonist induced down-regulation, or to beta-globin mRNA; (iii) displays binding to beta2-adrenergic receptor mRNA that is selectively competed by poly(U) RNA, but not poly(A), -(C), or -(G) RNA; and (iv) displays binding to receptor mRNA that can be competed by RNA harboring destabilizer sequences that are AU-rich and AUUUA pentamer-rich. The abundance of the M(r) 35,000 RNA-binding protein selective for beta-adrenergic receptor message, a factor we term betaARB protein, varies inversely with the level of receptor mRNA, being induced by agonists that down-regulate receptor mRNA.