Histone H2B-associated proteins: The Arabidopsis nucleolin 1 binds H2B and facilitates nucleosome disassembly via RNA-dependent mechanism.

To gain insight into the function of histone H2B variants in Arabidopsis, we studied the histone H2B.9 variant, attempting to uncover its interacting proteins. Accordingly, nuclear extract derived from H2B.9-GFP-expressing plants was subjected to GFP-Trap followed by proteome analysis. This analysis revealed 106 H2B.9-associated proteins, among them splicing factors, chromatin remodeling factors, and nucleolar proteins, including the histone chaperone nucleolin 1 (AtNuc-L1). Like animal nucleolins, AtNuc-L1 is known for its function in rRNA transcription and ribosome biogenesis, as well as its role in chromatin organization and nucleosome sliding; yet, the molecular mechanism underlying its function remains unclear. We showed that it specifically binds histone H2B, but not other core histones, in the context of nucleosomes and facilitates disassembly of naturally occurring nucleosomes derived from tobacco leaves via chromatin fractionation. This activity requires additional nuclear factors and is dependent on binding RNA molecules. Thus, our findings reveal the magnitude of nuclear processes potentially mediated by histone H2B and provide evidence for the role of the H2B-interacting AtNuc-L1 in RNA-dependent nucleosome disassembly.