Revealing cis- and trans-regulatory elements underlying nuclear distribution and function of the Arabidopsis histone H2B.8 variant.

The H2B.8 variant has been diverged from other variants by its extended N-terminal region that possesses a conserved domain. We generated transgenic Arabidopsis plants expressing H2B.9 (class I), H2B.5 (class II) and H2B.8 (class III) fused to GFP under the 35 S promoter and studied their nuclear distribution and function. H2B.8-GFP showed peculiar nuclear localization at chromocenters in all cell types examined, while H2B.5-GFP and H2B.9-GFP displayed various patterns often dependent on cell types. H2B variants faithfully assembled onto nucleosomes showing no effect on nuclear organization; H2B.8-GFP appeared as three distinct isoforms in which one isoform appeared to be SUMOylated. Interestingly, transient expression in protoplasts revealed H2B.8 nuclear localization distinct from transgenic plants as it was restricted to the nuclear periphery generating a distinctive ring-like appearance accompanied by nuclear size reduction. This unique appearance was abolished by deletion of the N-terminal conserved domain or when H2B.8-GFP is transiently expressed in ddm1 protoplasts. GFP-TRAP-coupled proteome analysis uncovered H2B.8-partner proteins including H2A.W.12, which characterizes heterochromatin. Thus, our data highlight H2B.8 as a unique variant evolved in angiosperms to control chromatin compaction/aggregation and uncover cis- and trans-regulatory elements underlying its nuclear distribution and function.