Histone H2A serine-1 phosphorylation is a chaperone-dependent signal for dimerization with H2B and for enhanced deposition

Multiple histone chaperones and histone modifications are involved in the folding, transport, and re-lease of histones onto newly replicated DNA. Little is known about histone H2A-H2B pre-deposition his-tone modifications and their regulation of histone deposition. We previously showed that H2A serine 1 phosphorylation (H2AS1ph) is enriched on the soluble egg histones and on zygotic chromatin in Xenopus embryos. Here, we demonstrate that H2AS1 phosphorylation is required for a timely incorporation of H2A-H2B into the pronuclear chromatin. Our analysis revealed that exogenous H2AS1A-H2B dimers were poorly incorporated into pronuclei in egg extract compared with wildtype and H2AS1E-H2B dimers. Chaperone-mediated deposition using histones purified from pronuclei showed that neither Nap1 nor Nucleoplasmin (Npm2) histone deposition was directly affected by endogenous histone posttranslational modification. We further demonstrate that H2AS1 phosphorylation was dependent on Npm2 and required H2B. Surprisingly, Nap1 was incapable of promoting H2AS1 phosphorylation. These results suggest that serine 1 phosphorylation signals a specific state of H2A-H2B dimer bound by Nucleoplasmin. Neither Npm2 nor Nap1 exhibited preference for binding H2AS1A or H2AS1E mutant histones or dimers with H2B in vitro. We propose that H2AS1 phosphorylation is a pre-deposition modification that signals for the proper dimerization of H2A-H2B, which in turn activates downstream effectors leading to H2A-H2B deposition.