Rapidly and transiently inhibited by phosphorylation following the generation of DNA double-stranded breaks during S-phase. This is cell cycle checkpoint and ATM-pathway dependent and appears to regulate processes involved in chromatin assembly. Isoform 3 phosphorylates and enhances the stability of the t-SNARE SNAP23, augmenting its assembly with syntaxin. Isoform 3 protects the cells from the ionizing radiation by facilitating the repair of DSBs. In vitro, phosphorylates histone H3 at 'Ser-10'.Yamakawa A., Gene 202:193-201(1997).Sillje H.H.W., EMBO J. 18:5691-5702(1999).Cabaniols J.-P., Mol. Biol. Cell 10:4033-4041(1999).