Progress and outlook in studying the substrate specificities of PARPs and related enzymes.

Suskiewicz, Marcin J; Palazzo, Luca; Hughes, Rebecca; Ahel, Ivan

Despite decades of research on ADP-ribosyltransferases (ARTs) from the poly(ADP-ribose) polymerase (PARP) family, one key aspect of these enzymes-their substrate specificity-has remained unclear. Here we briefly discuss the history of this area and, more extensively, the recent breakthroughs, including the identification of protein serine residues as a major substrate of PARP1 and PARP2 in human cells and of cysteine and tyrosine as potential targets of specific PARPs. On the molecular level, the modification of serine residues requires a composite active site formed by PARP1 or PARP2 together with a specificity-determining factor, HPF1; this represents a new paradigm not only for PARPs but generally for post-translational modification (PTM) catalysis. Additionally, we discuss the identification of DNA as a substrate of PARP1, PARP2, PARP3, and some bacterial ARTs and the discovery of non-canonical RNA capping by several PARP family members. Together, these recent findings shed new light on PARP-mediated catalysis and caution to "expect the unexpected" when it comes to further potential substrates.