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.

Automated summary

Poly(ADP-ribose) (PAR) is a homopolymer that regulates cellular processes as a posttranslational modification and serves as a scaffold for protein binding in macromolecular complexes. This study demonstrates that PAR has greater stiffness and undergoes sharper compaction in response to cation binding compared to RNA and DNA. The degree of PAR compaction depends on the concentration and valency of cations, and the intrinsically disordered protein FUS can also induce PAR compaction. These findings suggest that a cationic environment may play a role in the recognition specificity of PAR.