Regardless of the deposition pathway, aminoacid 31 in histone variant H3 is essential at gastrulation in Xenopus

David Sitbon, Ekaterina Boyarchuk, Geneviève Almouzni

AbstractThe closely related replicative H3 and non-replicative H3.3 variants show specific requirement during development in vertebrates. Whether it involves distinct mode of deposition or unique roles once incorporated into chromatin remains unclear. To disentangle the two aspects, we took advantage of the Xenopus early development combined with chromatin assays. Our previous work showed that in Xenopus, depletion of the non-replicative variant H3.3 impairs development at gastrulation, without compensation through provision of the replicative variant H3.2. We systematically mutated H3.3 at each four residues that differ from H3.2 and tested their ability to rescue developmental defects. Surprisingly, all H3.3 mutated variants functionally complemented endogenous H3.3, regardless of their incorporation pathways, except for one residue. This particular residue, the serine at position 31 in H3.3, gets phosphorylated onto chromatin in a cell cycle dependent manner. While the alanine substitution failed to rescue H3.3 depletion, a phosphomimic residue sufficed. We conclude that the time of gastrulation reveals a critical importance of the H3.3S31 residue independently of the variant incorporation pathway. We discuss how this single evolutionary conserved residue conveys a unique property for this variant in vertebrates during cell cycle and cell fate commitment.