Couto et al. describe how a poly ADP-ribose polymerase (PARP) helps repair DNA double-strand breaks (DSBs) by promoting the accumulation of a key protein required for nonhomologous end joining (NHEJ).
PARPs are activated by DNA damage, catalyzing the poly ADP-ribosylation (PARylation) of proteins at both single-strand breaks and DSBs. In vertebrate cells, PARP1 and PARP2 regulate the repair of single-strand lesions and promote DSB repair by homologous recombination. But how PARylation promotes DSB repair by the NHEJ pathway—and which PARP is responsible—is unclear. Couto et al. investigated the functions of PARPs in Dictyostelium, a genetically tractable organism that expresses many DNA-repair proteins found in vertebrates but not lower organisms such as yeast.
Couto et al. discovered that two Dictyostelium PARPs, Adprt1b and Adprt2, were required to repair single-strand DNA lesions. PARylation at DSBs, however, was principally carried out by a third PARP, Adprt1a. The NHEJ pathway was less active in Dictyostelia lacking Adprt1a because the Ku complex, which recognizes DNA ends and recruits the NHEJ repair machinery, no longer accumulated on chromatin after DNA damage.
The Ku70 subunit contains a motif that binds to poly ADP-ribose. A version of Ku70 lacking this domain failed to accumulate on DNA after DSB induction and was unable to rescue the NHEJ defects of cells lacking wild-type Ku70. Senior author Nicholas Lakin now wants to identify which proteins are PARylated by Adprt1a in order to recruit or retain Ku70 at DSBs.