The C-terminal domain of p53 orchestrates the interplay between non-covalent and covalent poly(ADP-ribosyl)ation of p53 by PARP1.

The post-translational modification poly(ADP-ribosyl)ation (PARylation) plays key roles in genome maintenance and transcription. Both non-covalent poly(ADP-ribose) binding and covalent PARylation control protein functions, however, it is unknown how the two modes of modification crosstalk mechanistically. Employing the tumor suppressor p53 as a model substrate, this study provides detailed insights into the interplay between non-covalent and covalent PARylation and unravels its functional significance in the regulation of p53. We reveal that the multifunctional C-terminal domain (CTD) of p53 acts as the central hub in the PARylation-dependent regulation of p53. Specifically, p53 bound to auto-PARylated PARP1 via highly specific non-covalent PAR-CTD interaction, which conveyed target specificity for its covalent PARylation by PARP1. Strikingly, fusing the p53-CTD to a protein that is normally not PARylated, renders this a target for covalent PARylation as well. Functional studies revealed that the p53-PAR interaction had substantial implications on molecular and cellular levels. Thus, PAR significantly influenced the complex p53-DNA binding properties and controlled p53 functions, with major implications on the p53-dependent interactome, transcription, and replication-associated recombination. Remarkably, this mechanism potentially also applies to other PARylation targets, since a bioinformatics analysis revealed that CTD-like regions are highly enriched in the PARylated proteome.

α-Synuclein oligomers distinctively permeabilize complex model membranes.

α-Synuclein oligomers are increasingly considered to be responsible for the death of dopaminergic neurons in Parkinson's disease. The toxicity mechanism of α-synuclein oligomers likely involves membrane permeabilization. Even though it is well established that α-synuclein oligomers bind and permeabilize vesicles composed of negatively-charged lipids, little attention has been given to the interaction of oligomers with bilayers of physiologically relevant lipid compositions. We investigated the interaction of α-synuclein with bilayers composed of lipid mixtures that mimic the composition of plasma and mitochondrial membranes. In the present study, we show that monomeric and oligomeric α-synuclein bind to these membranes. The resulting membrane leakage differs from that observed for simple artificial model bilayers. Although the addition of oligomers to negatively-charged lipid vesicles displays fast content release in a bulk permeabilization assay, adding oligomers to vesicles with compositions mimicking mitochondrial membranes shows a much slower loss of content. Oligomers are unable to induce leakage in the artificial plasma membranes, even after long-term incubation. CD experiments indicate that binding to lipid bilayers initially induces conformational changes in both oligomeric and monomeric α-synuclein, which show little change upon long-term incubation of oligomers with membranes. The results of the present study demonstrate that the mitochondrial model membranes are more vulnerable to permeabilization by oligomers than model plasma membranes reconstituted from brain-derived lipids; this preference may imply that increasingly complex membrane components, such as those in the plasma membrane mimic used in the present study, are less vulnerable to damage by oligomers.

α-Synuclein oligomers: an amyloid pore? Insights into mechanisms of α-synuclein oligomer-lipid interactions.

In many human diseases, oligomeric species of amyloid proteins may play a pivotal role in cytotoxicity. Many lines of evidence indicate that permeabilization of cellular membranes by amyloid oligomers may be the key factor in disrupting cellular homeostasis. However, the exact mechanisms by which the membrane integrity is impaired remain elusive. One prevailing hypothesis, the so-called amyloid pore hypothesis, assumes that annular oligomeric species embed into lipid bilayers forming transbilayer protein channels. Alternatively, an increased membrane permeability could be caused by thinning of the hydrophobic core of the lipid bilayer due to the incorporation of the oligomers between the tightly packed lipids, which would facilitate the transport of small molecules across the membrane. In this review, we briefly recapitulate our findings on the structure of α-synuclein oligomers and the factors influencing their interaction with lipid bilayers. Our results, combined with work from other groups, suggest that α-synuclein oligomers do not necessarily form pore-like structures. The emerging consensus is that local structural rearrangements of the protein lead to insertion of specific regions into the hydrophobic core of the lipid bilayer, thereby disrupting the lipid packing.