Beta-adrenergic agonists that down-regulate receptor mRNA up-regulate a M(r) 35,000 protein(s) that selectively binds to beta-adrenergic receptor mRNAs.

In an effort to explore the molecular basis for agonist-induced destabilization of &adrenergic receptor mRNA, we investigated the nature of RNA-binding proteins both in untreated and agonist-treated DDTI-MF2 smooth muscle cells.Messenger RNAs for the (Ylb-9 &-, and &-adrenergic receptors as well as for &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, 35,000 radiolabeled protein(s) with the following characteristics was identified: (i) binds selectively to B1and Bz-adrenergic receptor mRNAs, both of which undergo agonist-induced down-regulation; (ii) does not bind to either (Ylb' adrenergic receptor mRNA, which does not undergo agonist induced down-regulation, or to &globin mRNA; (iii) displays binding to &-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 AUrich and AUUUA pentamer-rich.The abundance of the M, 35,000 RNA-binding protein selective for @-adrenergic receptor message, a factor we term BARB protein, varies inversely with the level of receptor mRNA, being induced by agonists that down-regulate receptor mRNA.Steady-state levels of &-adrenergic receptor protein and mRNA have been shown to decline in cells stimulated with &adrenergic agonists (Hadcock and Malbon, 1988b;Collins et al., 1991).Furthermore, protein kinase A activity has been shown to be obligate for the expression of agonist-promoted down-regulation of receptor mRNA levels (Hadcock et al., 1989a).