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Chattaraj A, Baltaci Z, Chung S, Mayer BJ, Loew LM, Ditlev JA. Measurement of solubility product reveals the interplay of oligomerization and self-association for defining condensate formation. Mol Biol Cell 2024; 35:ar122. [PMID: 39046778 PMCID: PMC11449392 DOI: 10.1091/mbc.e24-01-0030] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2024] [Revised: 06/06/2024] [Accepted: 07/16/2024] [Indexed: 07/25/2024] Open
Abstract
Cellular condensates often consist of 10s to 100s of distinct interacting molecular species. Because of the complexity of these interactions, predicting the point at which they will undergo phase separation is daunting. Using experiments and computation, we therefore studied a simple model system consisting of polySH3 and polyPRM designed for pentavalent heterotypic binding. We tested whether the peak solubility product, or the product of the dilute phase concentration of each component, is a predictive parameter for the onset of phase separation. Titrating up equal total concentrations of each component showed that the maximum solubility product does approximately coincide with the threshold for phase separation in both experiments and models. However, we found that measurements of dilute phase concentration include small oligomers and monomers; therefore, a quantitative comparison of the experiments and models required inclusion of small oligomers in the model analysis. Even with the inclusion of small polyPRM and polySH3 oligomers, models did not predict experimental results. This led us to perform dynamic light scattering experiments, which revealed homotypic binding of polyPRM. Addition of this interaction to our model recapitulated the experimentally observed asymmetry. Thus, comparing experiments with simulation reveals that the solubility product can be predictive of the interactions underlying phase separation, even if small oligomers and low affinity homotypic interactions complicate the analysis.
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Affiliation(s)
- Aniruddha Chattaraj
- R. D. Berlin Center for Cell Analysis and Modeling, University of Connecticut School of Medicine, Farmington, CT 06030
| | - Zeynep Baltaci
- Program in Molecular Medicine, Toronto, ON M5G 1E8, Canada
- Department of Biochemistry, University of Toronto, Toronto, ON M5S 1A1, Canada
| | - Steve Chung
- Program in Molecular Medicine, Toronto, ON M5G 1E8, Canada
| | - Bruce J. Mayer
- R. D. Berlin Center for Cell Analysis and Modeling, University of Connecticut School of Medicine, Farmington, CT 06030
- Genetics and Genome Sciences, University of Connecticut School of Medicine, Farmington, CT 06030
| | - Leslie M. Loew
- R. D. Berlin Center for Cell Analysis and Modeling, University of Connecticut School of Medicine, Farmington, CT 06030
| | - Jonathon A. Ditlev
- Program in Molecular Medicine, Toronto, ON M5G 1E8, Canada
- Program in Cell Biology, Hospital for Sick Children, Toronto, ON M5G 1E8, Canada
- Department of Biochemistry, University of Toronto, Toronto, ON M5S 1A1, Canada
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Chattaraj A, Baltaci Z, Mayer BJ, Loew LM, Ditlev JA. Measurement of solubility product in a model condensate reveals the interplay of small oligomerization and self-association. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.01.23.576869. [PMID: 38328089 PMCID: PMC10849621 DOI: 10.1101/2024.01.23.576869] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/09/2024]
Abstract
Cellular condensates often consist of 10s to 100s of distinct interacting molecular species. Because of the complexity of these interactions, predicting the point at which they will undergo phase separation into discrete compartments is daunting. Using experiments and computation, we therefore studied a simple model system consisting of 2 proteins, polySH3 and polyPRM, designed for pentavalent heterotypic binding. We tested whether the peak solubility product, the product of dilute phase monomer concentrations, is a predictive parameter for the onset of phase separation. Titrating up equal total concentrations of each component showed that the maximum solubility product does approximately coincide with the threshold for phase separation in both the experiments and models. However, we found that measurements of dilute phase concentration include contributions from small oligomers, not just monomers; therefore, a quantitative comparison of the experiments and models required inclusion of small oligomers in the model analysis. We also examined full phase diagrams where the model results were almost symmetric along the diagonal, but the experimental results were highly asymmetric. This led us to perform dynamic light scattering experiments, where we discovered a weak homotypic interaction for polyPRM; when this was added to the computational model, it was able to recapitulate the experimentally observed asymmetry. Thus, comparing experiments to simulation reveals that the solubility product can be predictive of phase separation, even if small oligomers and low affinity homotypic interactions preclude experimental measurement of monomer concentration.
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Affiliation(s)
- Aniruddha Chattaraj
- R. D. Berlin Center for Cell Analysis and Modeling, University of Connecticut School of Medicine, Farmington, Connecticut, United States of America
| | - Zeynep Baltaci
- Program in Molecular Medicine, Hospital for Sick Children, Toronto, Ontario, Canada
- Department of Biochemistry, University of Toronto, Toronto, Ontario, Canada
| | - Bruce J. Mayer
- R. D. Berlin Center for Cell Analysis and Modeling, University of Connecticut School of Medicine, Farmington, Connecticut, United States of America
- Genetics and Genome Sciences, University of Connecticut School of Medicine, Farmington, Connecticut, United States of America
| | - Leslie M. Loew
- R. D. Berlin Center for Cell Analysis and Modeling, University of Connecticut School of Medicine, Farmington, Connecticut, United States of America
| | - Jonathon A. Ditlev
- Program in Molecular Medicine, Hospital for Sick Children, Toronto, Ontario, Canada
- Department of Biochemistry, University of Toronto, Toronto, Ontario, Canada
- Program in Cell Biology, Hospital for Sick Children, Toronto, Ontario, Canada
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