Disentangling polydispersity in the PCNA−p15PAF complex, a disordered, transient and multivalent macromolecular assembly

• Published in: Nucleic acids research Vol. 45; no. 3; pp. 1501 - 1515
• Main Authors: Cordeiro, Tiago N., Chen, Po-chia, De Biasio, Alfredo, Sibille, Nathalie, Blanco, Francisco J., Hub, Jochen S., Crehuet, Ramon, Bernadó, Pau
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 17.02.2017
• Subjects: Biochemistry, Molecular Biology; Biological Physics; Carrier Proteins - chemistry; Chemical Sciences; Humans; Intrinsically Disordered Proteins - chemistry; Life Sciences; Molecular Dynamics Simulation; Multiprotein Complexes - chemistry; Nuclear Magnetic Resonance, Biomolecular; Physics; Proliferating Cell Nuclear Antigen - chemistry; Protein Structure, Quaternary; Recombinant Proteins - chemistry; Scattering, Small Angle; Structural Biology; X-Ray Diffraction
• ISSN: 0305-1048; 1362-4962
• DOI: 10.1093/nar/gkw1183

The intrinsically disordered p15PAF regulates DNA replication and repair when interacting with the Proliferating Cell Nuclear Antigen (PCNA) sliding clamp. As many interactions between disordered proteins and globular partners involved in signaling and regulation, the complex between p15PAF and trimeric PCNA is of low affinity, forming a transient complex that is difficult to characterize at a structural level due to its inherent polydispersity. We have determined the structure, conformational fluctuations, and relative population of the five species that coexist in solution by combining small-angle X-ray scattering (SAXS) with molecular modelling. By using explicit ensemble descriptions for the individual species, built using integrative approaches and molecular dynamics (MD) simulations, we collectively interpreted multiple SAXS profiles as population-weighted thermodynamic mixtures. The analysis demonstrates that the N-terminus of p15PAF penetrates the PCNA ring and emerges on the back face. This observation substantiates the role of p15PAF as a drag regulating PCNA processivity during DNA repair. Our study reveals the power of ensemble-based approaches to decode structural, dynamic, and thermodynamic information from SAXS data. This strategy paves the way for deciphering the structural bases of flexible, transient and multivalent macromolecular assemblies involved in pivotal biological processes.