Ubiquitylation, the modification of proteins with ubiquitin (Ub), is one of the most versatile post-translational modifications in eukaryotic cells. Since Ub also serves as its own substrate, proteins can be modified by numerous different Ub chains, in which the individual moieties are linked via one or several of the seven lysines of Ub. Homogeneous Ub chains, in which the moieties are sequentially linked via the same residue, have been most extensively studied. However, due to their restricted availability, the functions of Ub chains linked via K27, K29, or K33 are poorly understood. We have developed an approach that, for the first time, allows the generation of all seven homogeneous Ub chains in large quantities. The potential of our approach is demonstrated by the identification of previously unknown interaction partners of K27-, K29-, and K33-linked Ub chains by affinity-based proteomics.

Decoding the role of histone posttranslational modifications (PTMs) is key to understand the fundamental process of epigenetic regulation. This is well studied for PTMs of core histones but not for linker histone H1 in general and its ubiquitylation in particular due to a lack of proper tools. Here, we report on the chemical synthesis of site-specifically mono-ubiquitylated H1.2 and identify its ubiquitin-dependent interactome on a proteome-wide scale. We show that site-specific ubiquitylation of H1 at position K64 modulates interactions with deubiquitylating enzymes and the deacetylase SIRT1. Moreover, it affects H1-dependent chromatosome assembly and phase separation resulting in a more open chromatosome conformation generally associated with a transcriptionally active chromatin state. In summary, we propose that site-specific ubiquitylation plays a general regulatory role for linker histone H1.

Post‐translational modification (PTM) with ADP‐ribose and poly(ADP‐ribose) using nicotinamide adenine dinucleotide (NAD+) as substrate is involved in the regulation of numerous cellular pathways in eukaryotes, notably the response to DNA damage caused by cellular stress. Nevertheless, due to intrinsic properties of NAD+ e.g., high polarity and associated poor cell passage, these PTMs are difficult to characterize in cells. Here, two new NAD+ derivatives are presented, which carry either a fluorophore or an affinity tag and, in combination with developed methods for mild cell delivery, allow studies in living human cells. We show that this approach allows not only the imaging of ADP‐ribosylation in living cells but also the proteome‐wide analysis of cellular adaptation by protein ADP‐ribosylation as a consequence of environmental changes such as H2O2‐induced oxidative stress or the effect of the approved anti‐cancer drug olaparib. Our results therefore pave the way for further functional and clinical studies of the ADP‐ribosylated proteome in living cells in health and disease.

Linker histone H1 plays a key role in chromatin organization and maintenance, yet our knowledge of the regulation of H1 functions by post-translational modifications is rather limited. In this study, we report on the generation of site-specifically mono-and diacetylated linker histone H1.2 by genetic code expansion. We used these modified histones to identify and characterize the acetylation-dependent cellular interactome of H1.2 by affinity purification mass spectrometry and show that site-specific acetylation results in overlapping but distinct groups of interacting partners. Among these, we find multiple translational initiation factors and transcriptional regulators such as the NAD +dependent deacetylase SIRT1, which we demonstrate to act on acetylated H1.2. Taken together, our data suggest that site-specific acetylation of H1.2 plays a role in modulating protein−protein interactions.

The attachment of ubiquitin (Ub) chains of various length to proteins is a prevalent posttranslational modification in eukaryotes. The fate of a modified protein is determined by Ub‐binding proteins (UBPs), which interact with Ub chains in a linkage‐selective manner. However, the impact and functional consequences of chain length on the binding selectivity of UBPs remain mostly elusive. We have generated Ub chains of defined length and linkage by using click chemistry and GELFrEE fractionation. These defined polymers were used in affinity‐based enrichment assays to identify length‐ and linkage‐selective interaction partners on a proteome‐wide scale. For the first time, it is revealed that the length of a Ub chain generally has a major impact on its ability to be selectively recognized by UBPs.