Differential Enzymatic Activity of Rat ADAR2 Splicing Variants Is Due to Altered Capability to Interact with RNA in the Deaminase Domain

In mammals, adenosine (A) to inosine (I) RNA editing is performed by adenosine deaminases acting on RNA (ADAR), ADAR1 and ADAR2 enzymes, encoded by mRNAs that might undergo splicing process. In rat, two splicing events produce several isoforms of ADAR2, called ADAR2a, ADAR2b, ADAR2e, and ADAR2f, but only ADAR2a and ADAR2b are translated into an active protein. In particular, they differ for ten amino acids located in the catalytic domain of ADAR2b. Here, we focused on these two isoforms, analyzing the splicing pattern and their different function during rat neuronal maturation. We found an increase of editing levels in cortical neurons overexpressing ADAR2a compared to those overexpressing ADAR2b. These results indicate ADAR2a isoform as the most active one, as reported for the homologous human short variant. Furthermore, we showed that the differential editing activity is not due to a different dimerization of the two isoforms; it seems to be linked to the ten amino acids loop of ADAR2b that might interfere with RNA binding, occupying the space volume in which the RNA should be present in case of binding. These data might shed light on the complexity of ADAR2 regulations.