SiMPull-POP: Quantification of Membrane Protein Assembly via Single Molecule Photobleaching

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Traditional methods for studying protein-protein interactions lack resolution for quantitative analysis of distinct oligomeric states, especially for membrane proteins in native lipid environments.
• Ensemble techniques like co-immunoprecipitation and FRET average heterogeneous populations, obscuring the precise stoichiometry of protein complexes.

Purpose of the Study:

• To develop a high-sensitivity single-molecule technique, SiMPull-POP (single-molecule pull-down polymeric nanodisc photobleaching), for quantifying membrane protein oligomerization in a near-native context.
• To enable precise, quantitative analysis of membrane protein assembly by preserving native lipid interactions using diisobutylene maleic acid (DIBMA) nanodiscs.

Main Methods:

• SiMPull-POP utilizes photobleaching at the single-molecule level to resolve monomeric, dimeric, and higher-order oligomeric states.
• Membrane proteins are incorporated into diisobutylene maleic acid lipid particles (DIBMALPs) to maintain native lipid interactions.
• The technique was validated using model systems including Omp25, a membrane-tethered FKBP protein, and the receptor tyrosine kinase EphA2.

Main Results:

• SiMPull-POP precisely determined the stoichiometry of membrane proteins, distinguishing monomeric, dimeric, and oligomeric states.
• A membrane-tethered FKBP protein showed ligand-dependent dimerization, while EphA2 shifted to higher-order oligomers upon Ephrin-A1 binding.
• Reduced membrane cholesterol content induced spontaneous EphA2 oligomerization, indicating cholesterol's role in suppressing receptor self-association.

Conclusions:

• SiMPull-POP offers significant advantages over conventional techniques, providing quantitative, single-molecule resolution of membrane protein complexes in a native-like environment.
• The method provides critical insights into how membrane properties and external stimuli regulate protein assembly, aiding the understanding of membrane protein function in health and disease.
• SiMPull-POP is a powerful tool for investigating both constitutive and regulated protein interactions, sensitive to ligand-induced or membrane property-driven changes in oligomerization.