Regulation of ADP-ribosyltransferase activity by ART domain dimerization in PARP15

Abstract

PARP15 is a mono-ADP-ribosyltransferase with unknown functions. Its evolutionary relationship with PARP14 suggests roles in antiviral defense; its ability to modify RNA and localization to stress granules point to functions in the regulation of translation. As many other PARP family members, PARP15 will readily automodify in vitro. PARP15 also contains two macrodomains predicted to bind ADP-ribosyl on targets. We used biochemical and biophysical analysis to study how the ADP-ribosyltransferase activity of PARP15 is regulated. Here we show that the catalytic domain of PARP15 dimerizes with mid-nanomolar affinity, forming the same dimer interface in solution that had already been captured by X-ray crystallography of the domain. Furthermore, we show that the formation of dimers is a prerequisite for catalytic activity and that monomeric mutant variants of the domain were catalytically inactive. Our findings suggest a regulatory mechanism by which dimerization activates either target engagement or placement of a catalytic residue, rather than NAD+ co-substrate binding, and by which the two protomers of the dimer operate independent of one another. Together, our results uncover a novel mechanism of regulation in a PARP family enzyme, which might inspire new avenues of pharmacological intervention.