SERious Surprises for ADP-Ribosylation Specificity: HPF1 Switches PARP1 Specificity to Ser Residues

In this issue of Molecular Cell, Bonfiglio et al. (2017) demonstrate that histone PARylation factor 1 (HPF1) is required for PARP1 to attach ADP-ribose groups onto the hydroxyl oxygen of the Ser residues of target substrates, including both PARP1 itself and histones. Here, mechanisms and implications of this unexpected, O-linked ADP-ribosylation are speculated on.