H1 provides the missing link

Abstract

npg Adding to the Histone Code at DNA double-strand breaks, Mailand and colleagues now uncover nondegradative ubiquitin marks on linker histone H1 as key signaling intermediates in the DNA damage signal transduction cascade.Ever since the original observation that histone H2AX mobilizes DNA repair factors into nuclear foci [1], the inception of a "Histone Code" in orchestrating DNA damage signal transduction and repair processes has sparked immense interests to define histone modifications at DNA damage sites.Cells utilize a growing collection of histone modifications to coordinate chromatin responses at DNA doublestrand breaks (DSBs), one of the most cytotoxic forms of DNA lesions. In particular, ubiquitin adducts deposited at the vicinity of DSBs provide docking sites for a cohort of DNA repair factors. Key to the formation of these DSBassociated ubiquitin adducts are the E3 ubiquitin ligases RNF8 and RNF168 [2][3][4][5][6][7]. RNF8 and RNF168 docking at DSBs follow a hierarchical sequence, and while the pair of E3 enzymes have been envisaged to, in concerted efforts, assemble DNA repair factors by ubiquitylating core histone proteins, the molecular basis that underlies their strict order of appearance at DSBs has remained elusive.RNF168, also known as the RIDDLE syndrome protein, was first described in a patient with radiosensitivity and immunodeficiency, clinical manifestations that are characteristic of defective responses to DSBs [6,8]. In addition to possessing a signature RING domain, the E3 ubiquitin ligase RNF168 also harbors two clusters of protein-interacting motifs (the authors referred to these as UDM1 and UDM2; see Figure 1) that supported its accrual at DNA damage sites [6,7,9,10]. Notably, although both UDMs were each endowed with ubiquitin-binding properties, they were unique in having evolved to target specialized ubiquitin structures at DSBs. Indeed, while UDM2 specifically interacted with ubiquitylated H2A histones, division of labor has it that UDM1 was important in bringing RNF168 to DSBs via an hitherto unknown factor [9].Notwithstanding the formidable nature in singling out this factor amongst the multiplicity of ubiquitin structures at DSBs, the Mailand team embarked on the mission to tease out exactly how RNF168 migrates to DSBs [11]. Because RNF168 is recruited to DSBs via an RNF8-UBC13-dependent ubiquitylation process [6,7], and the E3-E2 pair encoded the major K63-based ubiquitylating activity at DSBs, the authors took an unbiased proteomics approach and quantitatively compared the abundance of K63-ub conjugates in control cells with those that have been exposed to ionizing radiation (IR). To do so, they utilized an ubiquitin-binding cassette that bears remarkable specificity for K63-based ubiquitin polymers, and fished out chromatin factor(s) that showed enrichment after IR treatment. Much to anyone's surprise, linker histones, and not core histone proteins, were major bona fide targets of K63-linked ubiquitylation reactions! To examine whether K63-ubiquitylated H1 was duly respons...

Molecular and Cellular Functions of the Linker Histone H1.2

“…The ubiquitin-dependent DNA damage signal transduction pathway accounts for a key mechanism in regulating DNA damage response ( Jackson and Durocher, 2013 ). Ubiquitylation of H1.2 is suggested to be an important intermediate step for DSB repair mediated by the E3 ubiquitin ligases RNF168 and RNF8 ( Huen and Chen, 2016 ; Thorslund et al, 2015 ). DNA double-strand breaks trigger non-proteolytic ubiquitylation (K63-linked) proteins in adjacent chromatin areas to generate binding sites for DNA repair factors.…”

ITCH nuclear translocation and H1.2 polyubiquitination negatively regulate the DNA damage response

“…The ubiquitin (Ub)-dependent DNA damage signaling cascade is an important regulatory mechanism of the DDR (10). Polyubiquitinated histone H1 was recently shown to serve as an important signaling intermediate for the DSB repair process that depends on the E3 Ub ligases RNF8 and RNF168 (11,12). Whether the activity of polyubiquitinated histone H1 and RNF8/RNF168-dependent DDR events are negatively regulated in aggressive tumors, however, has not yet been explored.…”