Human UBN1 Is an Ortholog of Yeast Hpc2p and Has an Essential Role in the HIRA/ASF1a Chromatin-Remodeling Pathway in Senescent Cells

Abstract

Cellular senescence is an irreversible proliferation arrest, tumor suppression process and likely contributor to tissue aging. Senescence is often characterized by domains of facultative heterochromatin, called senescenceassociated heterochromatin foci (SAHF), which repress expression of proliferation-promoting genes. Given its likely contribution to tumor suppression and tissue aging, it is essential to identify all components of the SAHF assembly pathway. Formation of SAHF in human cells is driven by a complex of histone chaperones, namely, HIRA and ASF1a. In yeast, the complex orthologous to HIRA/ASF1a contains two additional proteins, Hpc2p and Hir3p. Using a sophisticated approach to search for remote orthologs conserved in multiple species through evolution, we identified the HIRA-associated proteins, UBN1 and UBN2, as candidate human orthologs of Hpc2p. We show that the Hpc2-related domain of UBN1, UBN2, and Hpc2p is an evolutionarily conserved HIRA/Hir-binding domain, which directly interacts with the N-terminal WD repeats of HIRA/Hir. UBN1 binds to proliferation-promoting genes that are repressed by SAHF and associates with histone methyltransferase activity that methylates lysine 9 of histone H3, a site that is methylated in SAHF. UBN1 is indispensable for formation of SAHF. We conclude that UBN1 is an ortholog of yeast Hpc2p and a novel regulator of senescence.

O-linked N -acetylglucosamine transferase (OGT) interacts with the histone chaperone HIRA complex and regulates nucleosome assembly and cellular senescence

“…To determine which subunit interacted with OGT, we first used HIRA mutants to define the OGT-HIRA proteinprotein interaction domain in vivo. HIRA contains three domains: the HIRA A region (residues 1-400) that consists of WD40 repeats and interacts with the N terminus of UBN1 (49,50); the HIRA B domain (residues 401-500) that interacts with ASF1a (33, 51, 52); and the HIRA C domain (residues 501-1017) that binds CABIN1 (28) (Fig. 1D, Top).…”

“…One possibility is that O-GlcNAc modification of HIRA influences the expression of the HIRA complex. However, unlike depletion of HIRA complex subunits that affects the stability of other subunits (28,49), depletion of OGT does not affect the expression of three HIRA subunits (Fig. 3 A and B).…”

The double face of the histone variant H3.3

“…HIRA has two orthologs in Saccharomyces cerevisiae, Hir1p and Hir2p, and biochemical purification of the Hir complex in this species revealed the presence of two co-purifying proteins Hir3 and Hpc2. Interestingly, their corresponding orthologs in human, Cabin1 and Ubinuclein (UBN1 and 2), respectively [63,64], co-purify with human H3.3 in a sub-complex with HIRA, showing that the yeast Hir complex is conserved in human [57,65]. It is thus tempting to speculate that HIRA is likely to function as a complex in the process of H3.3 deposition although the respective role of each protein is still unclear.…”

Chromatin maintenance and dynamics in senescence: a spotlight on SAHF formation and the epigenome of senescent cells

“…In addition, it was shown that knockdown of Asf1a by RNAi impairs OIS induced by overexpression of H--RasV12 (Zhang et al 2005). other members of the HIRA complex, namely UBN1 and Cabin1 were also shown to localize to PML--NBs only in senescent cells, and ectopic expression of these two proteins also led to SAHF formation and senescence entry, suggesting a function of the entire HIRA histone chaperone complex in SAHF assembly (Banumathy et al 2009;). Thus, these data suggested that the H3.3 histone chaperones forming the HIRA complex localize to PML--NBs during senescence and that this is important for SAHF formation (Fig.…”