Dimerization of human PARP15 is required for NAD⁺ binding and automodification

PARP proteins are enzymes catalyzing ADP-ribosylation, a well-conserved post-translational modification. In addition to the catalytic domain present in all PARPs, these proteins have a wide variety of other domains possessing different functions. Additional domains present in human PARP15 are macrodomains, capable of binding ADP-ribose. PARP15 is the least studied of the macrodomain-containing human PARPs, and it is linked to different diseases from infections to cancer. We show that the full-length canonical isoform 1 of PARP15 auto-ADP-ribosylates robustly on glutamate and aspartate residues, from which the modification can be hydrolyzed by known ADP-ribosylhydrolases. We have been able to locate several modification sites in the region of tandem macrodomains. Consistent with previous findings that the catalytic domain dimerization is necessary for enzymatic activity, we confirm this in the full-length context with the recombinant isoform 1 of PARP15. We show that dimerization is required for the efficient substrate NAD⁺ binding, and we provide structural basis for this requirement with the help of a co-crystal structure of the catalytic domain dimer with an unhydrolyzable substrate analog.