Nuclear porcupine mediates XRCC6/Ku70 S-palmitoylation in the DNA damage response

The activation of the DNA damage response (DDR) heavily relies on post-translational modifications (PTMs) of proteins, which play a crucial role in the prevention of genetic instability and tumorigenesis. Among these PTMs, palmitoylation is a highly conserved process that is dysregulated in numerous cancer types. However, its direct involvement in the DDR and the underlying mechanisms remain unclear.

CRISPR-Cas9 technology was used to generate the PORCN KO and PORCN NLS KO cell lines. The effects of PORCN NLS in the DDR were verified by colony formation assays, MTT assays, the DR/EJ5 homologous recombination/non-homologous end-joining reporter system, xenograft tumor growth and immunofluorescence. Mechanisms were explored by mass spectrometry, acyl-biotin exchange (ABE) palmitoylation assay, Click-iT assay, cell subcellular fractionation assay, Western blot analysis, and in vivo and in vitro co-immunoprecipitation.

In this study, we introduce evidence that Porcupine (PORCN) is an integral component of and plays a critical role in the DDR. PORCN deficiency hampers nonhomologous end joining (NHEJ) and highly sensitizes cells to ionizing radiation (IR) both in vitro and in vivo. We also provide evidence that PORCN possesses a nuclear fraction (nPORCN) with S-acyltransferase activity, unlike its membrane-bound O-acyltransferase in the endoplasmic reticulum. Furthermore, we show that nPORCN is necessary for the successful activation of NHEJ. Using mass spectrometry, we reveal the existence of an nPORCN complex and show that nPORCN mediates the S-palmitoylation of XRCC6/Ku70 at five specific cysteine sites in response to IR. Mutation of these sites causes a substantial increase in radiosensitivity and delays NHEJ. Additionally, we present evidence that nPORCN-dependent Ku70 palmitoylation is required for DNA-PKcs/Ku70/Ku80 complex formation.

Our findings underscore the crucial role of nPORCN-dependent Ku70 S-palmitoylation in the DDR.