Control of human papillomavirus type 1 late mRNA stability and translation by an AU-rich RNA element

Sammanfattning: Human Papillomaviruses (HPVs) infect the epithelial tissues in humans and infection with certain high-risk types is associated with cancer. The expressionof the HPV late capsid genes is dependent on cell differentiation. This may be one reason for the lack of HPV replication in vitro.Here we have studied an AU-rich sequence in the 3´ untranslated region (UTR) on the HPV type 1 (HPV-1) late mRNAs that acts in cis to posttranscriptionally inhibit HPV late gene expression in undifferentiated cells. Mapping revealed that the minimal AU-rich element (ARE) was 57 nts long, 93% AU-rich and that point mutations in two AUUUA- and three UUUUU-motifs inactivated the ARE. Analysis of the mRNA stability in HeLa cells showed that mRNAs containing the HPV-1 ARE (h1ARE) had reduced half life. The h1ARE also acted by suppressing translation independently of the effect on mRNA stability. Inhibition of translation appeared to occur by suppressing a function mediated by the 3´-poly(A) tails.Analysis of RNA-protein interactions in vitro revealed that both cytoplasmic and nuclear proteins interacted specifically with the h1ARE. Three cellular proteins were identified as the heterogeneous nuclear ribonucleoproteins (hnRNPs) C1 and C2 and the HuR protein. Recombinant HuR protein bound specifically to the AUUUA- and UUUUU-sites within the h1ARE and the apparent Kd value of HuR binding to the h1ARE or the inactive mutants thereof differed more than 50 fold. Binding of HuR and hnRNPC1/C2 to the AUUUA- and UUUUU-motifs suggested their importance for the function of the h1ARE. The h1ARE was the major negative element on HPV-1 late mRNAs whereas HPV-16 late mRNAs contained multiple negative elements. The number of negative elements correlated with the levels of virions produced by these HPV types in vivo.