The exchange of H2A (green) into and out of chromatin can be seen in permeablized cells.
Histone assembly into chromatin is often studied in vivo by using fluorescently tagged proteins. But these studies rely on making guesses about what machinery might be involved. In vitro systems are less biased but generally lack the structural complexity and heterogeneity of natural chromatin.
Kimura's group got the best of both worlds by using permeabilized cells, which maintain their natural chromatin structure. Yet the system allowed the authors to purify unanticipated histone assembly and exchange factors, including protein phosphatase 2Cγ (PP2Cγ), which inserted histone H2A and H2B into preassembled nucleosomes.
PP2Cγ also dephosphorylated H2A and H2B. As phosphates mark histones at DNA-damaged sites, they must be...
