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The core histones H3 and H4 are phosphorylated by the p21‐activated protein kinase Pak2. Pak2 is a serine/threonine kinase that is activated by small G proteins in response to moderate stress and by caspase 3 cleavage during apoptotic stress. The sites in histones 3 and 4 phosphorylated by Pak2 were identified by ESI/MS/MS. H4 was phosphorylated on two sites by Pak2. These phosphorylation sites were located in the globular core domain in the consensus sequence identified for Pak2, which was identified in studies with synthetic heptapeptides. In H3, there were 4 identified sites located in the N‐terminal tail and the globular core domain. These were non‐consensus sites dependent on upstream basic amino acids in the −4 to −8 positions, which were predicted based on studies with synthetic dodecapeptides. The sites phosphorylated on H4 in vitro by Pak2 were shown to be phosphorylated under conditions of moderate stress, when Pak2 is activated. The sites phosphorylated in H3 in vitro, were reported to be phosphorylated in vivo under a variety of conditions, including apoptosis. Thus, phosphorylation of histones 3 and 4 could play important roles in regulating chromosome activity/function.

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