A novel RNA pentaloop fold involved in targeting ADAR2.

Adenosine deaminases that act on RNA (ADARs) catalyze the site-specific conversion of adenosine to inosine in primary mRNA transcripts, thereby affecting coding potential of mature mRNAs. Structural determinants that define the adenosine moieties for specific ADARs-mediated deaminations are currently unknown. We report the solution structure of the central region of the human R/G stem-loop pre-mRNA, a natural ADAR2 substrate encoding the subunit B of the glutamate receptor (R/G site). The structure reveals that the GCU(A/C)A pentaloop that is conserved in mammals and birds adopts a novel fold. The fold is stabilized by a complex interplay of hydrogen bonds and stacking interactions. We propose that this new pentaloop structure is an important determinant of the R/G site recognition by ADAR2.