Protein ligands to HuR modulate its interaction with target mRNAs in vivo

AU-rich elements (AREs) present in the 3' untranslated regions of many prot ooncogene, cytokine, and lymphokine messages target them for rapid degradat ion. HuR, a ubiquitously expressed member of the ELAV (embryonic lethal abn ormal vision) family of RNA binding proteins, selectively binds AREs and st abilizes ARE-containing mRNAs in transiently transfected cells. Here, we id entify four mammalian proteins that bind regions of HuR known to be essenti al for its ability to shuttle between the nucleus and the cytoplasm and to stabilize mRNA: SET alpha, SET beta pp32, and acidic protein rich in leucin e (APRIL), Three have been reported to be protein phosphatase 2A inhibitors . All four ligands contain long, acidic COOH-terminal tails. while pp32 and APRIL share a second motif: rev-like leucine-rich repeats in their NH2-ter minal regions. We show that pp32 and APRIL are nucleocytoplasmic shuttling proteins that interact with the nuclear export factor CRM1 (chromosomal reg ion maintenance protein 1). The inhibition of CRM1 by leptomycin B leads to the nuclear retention of pp32 and APRIL, their increased association with HuR, and an increase in HuR's association with nuclear poly(A)+ RNA. Furthe rmore, transcripts from the ARE-containing c-fos gene are selectively retai ned in the nucleus, while the cytoplasmic distribution of total poly(A)+ RN A is not altered. These data provide evidence that interaction of its ligan ds with HuR modulate HuRs ability to bind its target mRNAs in vivo and sugg est that CRM1 is instrumental in the export of at least some cellular mRNAs under certain conditions. We discuss the possible role of these ligands up stream of HuR in pathways that govern the stability of ARE-containing mRNAs .