UBR7 Acts as a Chaperone for Post-Nucleosomal Histone H3

Histone chaperones modulate the stability of histones beginning from histone synthesis, through incorporation into DNA, and during recycling during transcription and replication. Following histone removal from DNA, chaperones regulate histone storage and degradation. Here we characterize UBR7 as a histone H3.1 chaperone that modulates the supply of pre-existing post-nucleosomal histone complexes. UBR7 is a largely nuclear soluble protein. We demonstrate that UBR7 binds to post-nucleosomal H3K4me3 and H3K9me3 histones via its UBR box and PHD. UBR7 binds to the non-nucleosomal histone chaperone NASP. The pool of NASP-bound post-nucleosomal histones accumulate and chromatin is depleted of H3K4me3 nucleosomes in the absence of UBR7. We propose that the interaction of UBR7, NASP, and histones opposes the histone storage functions of NASP as UBR7 promotes reincorporation of post-nucleosomal H3 complexes. ChIP-Seq for GFP in GFPUBR7-293T cells (n=1), ATAC-Seq (n=2), CUT&RUN for H3K4me3 (n=1 of 2 antibodies), and RNA-Seq (n=3) in UBR7KO and parental 293T cells.