β-Adrenergic agonists that down-regulate receptor mRNA up-regulate a M_r 35,000 protein(s) that selectively binds to β-adrenergic receptor mRNAs

In an effort to explore the molecular basis for agonist-induced destabilization of β-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 α_1b-, β₁-, and β₂-adrenergic receptors as well as for β-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 β₁- and β₂-adrenergic receptor mRNAs, both of which undergo agonist-induced down-regulation; (ii) does not bind to either α_1b-adrenergic receptor mRNA, which does not undergo agonist induced down-regulation, or to β-globin mRNA; (iii) displays binding to β₂-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 β-adrenergic receptor message, a factor we term βARB protein, varies inversely with the level of receptor mRNA, being induced by agonists that down-regulate receptor mRNA.