H2A.X N-terminal acetylation is a newly identified NAA40-mediated modification that is responsive to UV irradiation

N-terminal acetylation (Nt-Ac), mediated by N-terminal acetyltransferases (NATs) is one of the most abundant protein modifications occurring approximately in 80% of all eukaryotic proteins. In contrast to the broad spectrum NATs, the human N-alpha-acetyltransferase 40 (NAA40) is highly specific, currently known to Nt-acetylate only the two histone proteins H4 and H2A, which share an Ser(1)-Gly(2)-Arg(3)-Gly(4) N-terminal sequence. Previous work from our lab and others has highlighted the biological and clinical relevance of this NAA40-mediated modification.

In this study, by performing in silico analysis of protein sequences combined with biochemical assays we identify the histone variants H2A.X and H2A.J and the chromatin remodeler SMARCD2 as new potential substrates of human NAA40. Subsequently, focusing on H2A.X, we show for the first time by mass spectrometry analysis that H2A.X is N-terminally acetylated (Nt-acH2A.X) within human cells. Next, we demonstrate that NAA40 specifically interacts and N-terminally acetylates histone H2A.X, in vitro and within cells. Finally, we provide evidence that H2A.X N-terminal acetylation is responsive to Ultraviolet B (UVB)-induced DNA damage and its associated enzyme NAA40 affects the survival of cells exposed to UVB irradiation.

Our findings identify H2A.X as a novel bona fide substrate of NAA40. Moreover, the responsiveness of H2A.X N-terminal acetylation to UV-induced DNA damage indicates that this is a dynamic modification with potential biological functions.