Body of Abstract: H2A.Z is a highly conserved variant of a canonical H2A histone found in all eukaryotes. In Humans, HsH2A.Z.1, and HsH2A.Z.2.1, together with HsH2A.Z.2.2, a brain-specific, alternatively spliced form of HsH2A.Z.2.1, are required for viability. In Arabidopsis, HTA8, HTA9, and HTA11 encode H2A.Z proteins, and loss of all three proteins is not deleterious to the plants.
Interestingly, Arabidopsis H2A.Z proteins share more amino acid identities with Human H2A.Zs than with other Arabidopsis H2A histones, indicating a high degree of H2A.Z conservation. Amino acids that vary between Arabidopsis and human H2A.Z are predominantly found at the N-terminal end. In humans, many N-terminal amino acids are known substrates for post-translational modifications that greatly affect H2A.Z-mediated gene regulation. Intriguing question is how much of functional conservation exists between human and Arabidopsis H2A.Zs considering that the most diverse parts of the proteins are at the N-terminal ends that harbor functionally significant amino acid residues.
A CRISPR-generated h2a.z null Arabidopsis plants were transformed with three Human H2A.Zs, as well as with Arabidopsis HTA11, to assess the ability of transgenes to rescue h2a.z phenotypes. Transgenic plants carrying HsH2A.Z.1 and HsH2A.Z.2.1 were phenotypically indistinguishable from WT plants or HTA11 transgenic plants, indicating a high degree of functional conservation between Human and Arabidopsis H2A.Zs. Interestingly, HsH2A.Z.2.2 was deleterious to transgenic plants as none of the 26 T1 plants survived. ChIP-seq and RNA-seq analyses revealed that HTA11 and HsH2A.Z1 transgenic plants restored more than 93 % of H2A.Z deposition and more than 85% of gene expression when compared to WT plants. Considering that the N-terminal ends of Arabidopsis and Human H2A.Zs are significantly different, and the fact that Human H2A.Z proteins almost fully restore the phenotypic and molecular defects of h2a.z null plants, we suggest that the N-terminal end of Arabidopsis H2A.Z is not crucial for its function.