Geometrical constraints limiting the poly(ADP-ribose) conformation investigated by molecular dynamics simulation

Length-dependent Intramolecular Coil-to-Globule Transition in Poly(ADP-ribose) Induced by Cations

Poly(ADP-ribose) (PAR), as part of a post-translational modification, serves as a flexible scaffold for noncovalent protein binding. Such binding is influenced by PAR chain length through a mechanism yet to be elucidated. Structural insights have been elusive, partly due to the difficulties associated with synthesizing PAR chains of defined lengths. Here, we employ an integrated approach combining molecular dynamics (MD) simulations with small-angle X-ray scattering (SAXS) experiments, enabling us to identify highly heterogeneous ensembles of PAR conformers at two different, physiologically relevant lengths: PAR 15 and PAR 22 . Our findings reveal that numerous factors including backbone conformation, base stacking, and chain length contribute to determining the structural ensembles. We also observe length-dependent compaction of PAR upon the addition of small amounts of Mg 2+ ions, with the 22-mer exhibiting ADP-ribose bundles formed through local intramolecular coil-to-globule transitions. This study illuminates how such bundling could be instrumental in deciphering the length-dependent action of PAR.

GRAPHICAL ABSTRACT

Switch-like compaction of poly(ADP-ribose) upon cation binding

Poly(ADP-ribose) (PAR) is a homopolymer of adenosine diphosphate ribose that is added to proteins as a posttranslational modification to regulate numerous cellular processes. PAR also serves as a scaffold for protein binding in macromolecular complexes, including biomolecular condensates. It remains unclear how PAR achieves specific molecular recognition. Here, we use single-molecule fluorescence resonance energy transfer (smFRET) to evaluate PAR flexibility under different cation conditions. We demonstrate that, compared to RNA and DNA, PAR has a longer persistence length and undergoes a sharper transition from extended to compact states in physiologically relevant concentrations of various cations (Na+, Mg2+, Ca2+, and spermine4+). We show that the degree of PAR compaction depends on the concentration and valency of cations. Furthermore, the intrinsically disordered protein FUS also served as a macromolecular cation to compact PAR. Taken together, our study reveals the inherent stiffness of PAR molecules, which undergo switch-like compaction in response to cation binding. This study indicates that a cationic environment may drive recognition specificity of PAR.

Switch-like Compaction of Poly(ADP-ribose) Upon Cation Binding

Poly(ADP-ribose) (PAR) is a homopolymer of adenosine diphosphate ribose that is added to proteins as a post-translational modification to regulate numerous cellular processes. PAR also serves as a scaffold for protein binding in macromolecular complexes, including biomolecular condensates. It remains unclear how PAR achieves specific molecular recognition. Here, we use single-molecule fluorescence resonance energy transfer (smFRET) to evaluate PAR flexibility under different cation conditions. We demonstrate that, compared to RNA and DNA, PAR has a longer persistence length and undergoes a sharper transition from extended to compact states in physiologically relevant concentrations of various cations (Na ⁺ , Mg ²⁺ , Ca ²⁺ , and spermine). We show that the degree of PAR compaction depends on the concentration and valency of cations. Furthermore, the intrinsically disordered protein FUS also served as a macromolecular cation to compact PAR. Taken together, our study reveals the inherent stiffness of PAR molecules, which undergo switch-like compaction in response to cation binding. This study indicates that a cationic environment may drive recognition specificity of PAR.

Significance

Poly(ADP-ribose) (PAR) is an RNA-like homopolymer that regulates DNA repair, RNA metabolism, and biomolecular condensate formation. Dysregulation of PAR results in cancer and neurodegeneration. Although discovered in 1963, fundamental properties of this therapeutically important polymer remain largely unknown. Biophysical and structural analyses of PAR have been exceptionally challenging due to the dynamic and repetitive nature. Here, we present the first single-molecule biophysical characterization of PAR. We show that PAR is stiffer than DNA and RNA per unit length. Unlike DNA and RNA which undergoes gradual compaction, PAR exhibits an abrupt switch-like bending as a function of salt concentration and by protein binding. Our findings points to unique physical properties of PAR that may drive recognition specificity for its function.

Poly(ADP-ribose) in Condensates: The PARtnership of Phase Separation and Site-Specific Interactions

Biomolecular condensates are nonmembrane cellular compartments whose formation in many cases involves phase separation (PS). Despite much research interest in this mechanism of macromolecular self-organization, the concept of PS as applied to a live cell faces certain challenges. In this review, we discuss a basic model of PS and the role of site-specific interactions and percolation in cellular PS-related events. Using a multivalent poly(ADP-ribose) molecule as an example, which has high PS-driving potential due to its structural features, we consider how site-specific interactions and network formation are involved in the formation of phase-separated cellular condensates.

A sePARate phase? Poly(ADP-ribose) versus RNA in the organization of biomolecular condensates

Condensates are biomolecular assemblies that concentrate biomolecules without the help of membranes. They are morphologically highly versatile and may emerge via distinct mechanisms. Nucleic acids-DNA, RNA and poly(ADP-ribose) (PAR) play special roles in the process of condensate organization. These polymeric scaffolds provide multiple specific and nonspecific interactions during nucleation and 'development' of macromolecular assemblages. In this review, we focus on condensates formed with PAR. We discuss to what extent the literature supports the phase separation origin of these structures. Special attention is paid to similarities and differences between PAR and RNA in the process of dynamic restructuring of condensates during their functioning.

Poly(ADP-Ribosyl) Code Functions

Poly(ADP-ribosyl)ation plays a key role in cellular metabolism. Covalent poly(ADP-ribosyl)ation affects the activity of the proteins engaged in DNA repair, chromatin structure regulation, gene expression, RNA processing, ribosome biogenesis, and protein translation. Non-covalent PAR-dependent interactions are involved in the various types of cellular response to stress and viral infection, such as inflammation, hormonal signaling, and the immune response. The review discusses how structurally different poly(ADP-ribose) (PAR) molecules composed of identical monomers can differentially participate in various cellular processes acting as the so-called "PAR code." The article describes the ability of PAR polymers to form functional biomolecular clusters through a phase-separation in response to various signals. This phase-separation contributes to rapid spatial segregation of biochemical processes and effective recruitment of the necessary components. The cellular PAR level is tightly controlled by a network of regulatory proteins: PAR code writers, readers, and erasers. Impaired PAR metabolism is associated with the development of pathological processes causing oncological, cardiovascular, and neurodegenerative diseases. Pharmacological correction of the PAR level may represent a new approach to the treatment of various diseases.

Poly(ADP-ribose): A Dynamic Trigger for Biomolecular Condensate Formation

Poly(ADP-ribose) (PAR) is a nucleic acid-like protein modification that can seed the formation of microscopically visible cellular compartments that lack enveloping membranes, recently termed biomolecular condensates. These PAR-mediated condensates are linked to cancer, viral infection, and neurodegeneration. Recent data have shown the therapeutic potential of modulating PAR conjugation (PARylation): PAR polymerase (PARP) inhibitors can modulate the formation and dynamics of these condensates as well as the trafficking of their components - many of which are key disease factors. However, the way in which PARylation facilitates these functions remains unclear, partly because of our lack of understanding of the fundamental parameters of intracellular PARylation, including the sites that are conjugated, PAR chain length and structure, and the physicochemical properties of the conjugates. This review first introduces the role of PARylation in regulating biomolecular condensates, followed by discussion of current knowledge gaps, potential solutions, and therapeutic applications.

Mechanistic insight into the role of Poly(ADP-ribosyl)ation in DNA topology modulation and response to DNA damage

Genotoxic stress generates single- and double-strand DNA breaks either through direct damage by reactive oxygen species or as intermediates of DNA repair. Failure to detect and repair DNA strand breaks leads to deleterious consequences such as chromosomal aberrations, genomic instability and cell death. DNA strand breaks disrupt the superhelical state of cellular DNA, which further disturbs the chromatin architecture and gene activity regulation. Proteins from the poly(ADP-ribose) polymerase (PARP) family, such as PARP1 and PARP2, use NAD+ as a substrate to catalyse the synthesis of polymeric chains consisting of ADP-ribose units covalently attached to an acceptor molecule. PARP1 and PARP2 are regarded as DNA damage sensors that, upon activation by strand breaks, poly(ADP-ribosyl)ate themselves and nuclear acceptor proteins. Noteworthy, the regularly branched structure of poly(ADP-ribose) polymer suggests that the mechanism of its synthesis may involve circular movement of PARP1 around the DNA helix, with a branching point in PAR corresponding to one complete 360° turn. We propose that PARP1 stays bound to a DNA strand break end, but rotates around the helix displaced by the growing poly(ADP-ribose) chain, and that this rotation could introduce positive supercoils into damaged chromosomal DNA. This topology modulation would enable nucleosome displacement and chromatin decondensation around the lesion site, facilitating the access of DNA repair proteins or transcription factors. PARP1-mediated DNA supercoiling can be transmitted over long distances, resulting in changes in the high-order chromatin structures. The available structures of PARP1 are consistent with the strand break-induced PAR synthesis as a driving force for PARP1 rotation around the DNA axis.

Mutations of human DNA topoisomerase I at poly(ADP-ribose) binding sites: modulation of camptothecin activity by ADP-ribose polymers

Background

DNA topoisomerases are key enzymes that modulate the topological state of DNA through the breaking and rejoining of DNA strands. Human topoisomerase I belongs to the family of poly(ADP-ribose)-binding proteins and is the target of camptothecin derived anticancer drugs. Poly(ADP-ribosyl)ation occurs at specific sites of the enzyme inhibiting the cleavage and enhancing the religation steps during the catalytic cycle. Thus, ADP-ribose polymers antagonize the activity of topoisomerase I poisons, whereas PARP inhibitors increase their antitumor effects.

Methods

Using site-directed mutagenesis we have analyzed the interaction of human topoisomerase I and poly(ADP-ribose) through enzymatic activity and binding procedures.

Results

Mutations of the human topoisomerase I hydrophobic or charged residues, located on the putative polymer binding sites, are not sufficient to abolish or reduce the binding of the poly(ADP-ribose) to the protein. These results suggest either the presence of additional binding sites or that the mutations are not enough perturbative to destroy the poly(ADP-ribose) interaction, although in one mutant they fully abolish the enzyme activity.

Conclusions

It can be concluded that mutations at the hydrophobic or charged residues of the putative polymer binding sites do not interfere with the ability of poly(ADP-ribose) to antagonize the antitumor activity of topoisomerase I poisons.