Differential regulation of the histone chaperone HIRA during muscle cell differentiation by a phosphorylation switch

Replication-independent incorporation of variant histone H3.3 has a profound impact on chromatin function and numerous cellular processes, including the differentiation of muscle cells. The histone chaperone HIRA and H3.3 have essential roles in MyoD regulation during myoblast differentiation. However, the precise mechanism that determines the onset of H3.3 deposition in response to differentiation signals is unclear. Here we show that HIRA is phosphorylated by Akt kinase, an important signaling modulator in muscle cells. By generating a phosphospecific antibody, we found that a significant amount of HIRA was phosphorylated in myoblasts. The phosphorylation level of HIRA and the occupancy of phosphorylated protein on muscle genes gradually decreased during cellular differentiation. Remarkably, the forced expression of the phosphomimic form of HIRA resulted in reduced H3.3 deposition and suppressed the activation of muscle genes in myotubes. Our data show that HIRA phosphorylation limits the expression of myogenic genes, while the dephosphorylation of HIRA is required for proficient H3.3 deposition and gene activation, demonstrating that the phosphorylation switch is exploited to modulate HIRA/H3.3-mediated muscle gene regulation during myogenesis. A molecular switch controls muscle development Researchers in South Korea have identified a regulatory switch controlling when genes get turned on during muscle cell differentiation. Eun-Jung Cho of Sungkyunkwan University and co-workers investigated the regulation of a protein which attaches to DNA, called a histone, and activates genes during muscle development. The team analyzed the sequence of a helper protein which controls the attachment of the histone and identified a site which was enzymatically modified. The modification stops the helper protein from attaching the histone to muscle genes, delaying their activation. They showed that the level of this modification decreased during muscle development until it was low enough for histone attachment and gene activation. This molecular switch not only controls when muscle genes are activated but may also regulate other genes controlled by the same histone and helper protein.