analysis of the microfluidics coupled mass photometry data to investigate low-affinity protein-protein interactions

Expanding the Mass Photometry Analysis Concentration Range with Microfluidics

Protein-protein interactions underline most cellular functions, from immune regulation to DNA replication and translation. As a result, there is a fundamental need throughout the life sciences to investigate a vast range of interactions across diverse heterogenous complexes that are commonly formed. However, their detection, characterization, and quantification are often challenging, particularly for low-affinity interactions1.
Immunoprecipitation assays are often used to detect high-affinity interactions, but for low-affinity and transient interactions, detection is largely unfeasible2. Fluorescence techniques can also be used but require the potentially disruptive addition of fluorescent labels2. Cryo-EM can provide a structural snapshot and an ensemble readout of the protein complexes formed with high spatial resolution but also typically requires working at concentrations that are too low for imaging low-affinity interactions. Cryo-EM also brings challenges related to cost, accessibility, sample preparation, and analysis time3.
Additionally, surface plasmon resonance (SPR) has become a popular way to quantify protein-protein interactions, although it requires protein immobilization, which can affect the binding equilibrium and result in variable on-rates, thus reducing measurement accuracy4,5. It also involves several assay steps prior to data collection and analysis6.
Mass photometry is a single-molecule technique that has been used to analyze protein-protein interactions5,6,7. It works by measuring the mass of single molecules or complexes based on the light they scatter when they land on the surface of a glass coverslip8. Mass photometry measurements have been used to quantify binding affinities from the relative abundance of binding partners and the complexes they form5. Nevertheless, like other single-molecule techniques, the concentration of the sample to be measured should typically be less than 100 nM. If the concentration is higher, the molecules landing on the glass surface will overlap spatially, resulting in poor data quality7. Consequently, weaker interactions (KD ~micromolars), which dissociate at these lower concentrations, cannot be measured reliably since it is not possible to observe the necessary mixture of unbound and bound species5.
Here, we describe an approach that overcomes this limitation based on a new coupled microfluidics mass photometry device. Specifically, a microfluidics system is used in combination with the mass photometer to effectively expand the range of interactions that can be quantified by mass photometry. Microfluidics has been shown to offer a range of possibilities for investigating protein-protein interactions, including rapid dilution to detect weak interactions1,9. The system described herein operates by rapidly diluting the sample up to 10,000-fold on a microfluidic chip and immediately flowing it across the observation area of the chip, enabling the mass photometry measurement to start within 50 ms from when the molecules began the dilution process10. The dilution occurs when the sample and buffer are combined in a reverse Tesla valve mixer on the chip, with the relative flow rates of the two solutions determining the amount of dilution that occurs (see Protocol step 8). The flow rate is controllable with the microfluidic control software. Altering the flow rate can change the relative population of the species as it can impact the number of landing events on the surface of the glass, which is what is measured by the mass photometer.
The speed of the process is fast enough for the measurement to be completed before the integrity of the interaction has been disrupted (for further details, see also the Discussion). This can be understood through a brief look at the theory of first-order reactions, where <img alt="Equation 1" src="/files/ftp_upload/65772/65772eq01.jpg" style="margin: auto;" />. The forward (association) rate constant is kf, the backward (dissociation) rate constant is kb, and the equilibrium dissociation constant (KD) is defined as
KD = kb / kf
For protein binding, kf is generally limited by the reactants&#39; diffusion11 and so is restricted to the range of 106-107 M-1&#183;s-1. Because the range of is limited, a low-affinity (KD ~micromolars) reaction will have kb &#8776; 1 s-1. That is, kb = kf&#160;&#183; KD = (106 M-1&#183;s-1) (10-6 M) = 1 s-1, with the complex&#39;s half-life around 0.7 s11,12.
Our example system is the binding of the IgG monoclonal antibody trastuzumab to the soluble domain of the IgG neonatal Fc receptor (FcRn), which are known interacting partners13. Previously published data obtained using conventional mass photometry alone (i.e., with manual dilution of samples) showed that the proteins form multiple species. FcRn monomers, FcRn dimers, and unbound IgG were clearly visible, while IgG-FcRn complexes (at 1:1 and 1:2 ratios) were also detected (at pH 5.0) but only with very low abundance5. This observation prompts the question of whether IgG-FcRn complex formation could be more clearly detected if measured at a higher concentration. Indeed, combining mass photometry with a coupled rapid dilution approach described here provided more robust evidence of complex formation by an increase in their measured particles.
The mass photometry and microfluidics protocol described here makes it possible to characterize the formation of complexes with a KD up to the micromolar range. An empirical determination of the KD will require further improvements on the flow sensor accuracy, pump stability, chip-to-chip variations, and measurement location inside the observation window, as all these factors would influence the time from when the sample is diluted to being measured.
The same approach could be applied to investigate binding among any soluble proteins, provided they have distinct molecular weights (separated by at least 25 kDa) that fall into the range suitable for analysis with a mass photometer (30 kDa to 6 MDa). The insights obtained could be helpful for studies in a range of contexts - from gaining a mechanistic understanding of cellular functions to the design of new biotherapeutic drugs.

Author Spotlight: Characterization of Low-Affinity Protein Interactions in Solution Using MassFluidix Technology

Analysis of Protein Complex Formation at Micromolar Concentrations by Coupling Microfluidics with Mass Photometry

This protocol demonstrates a completely new way to analyze low affinity protein complexes. It allows researchers to detect and characterize certain protein interactions even when they are weak or transient, which has generally been very difficult or even impossible before. Protein interactions are crucial for cell function, but detecting and measuring them can be challenging.Mass photometry is a great tool for quantifying protein complex formation, but it has limitations with dilute samples. We're solving this problem with MassFluidix. The research gap we are addressing is how scientists can reliably characterize low affinity interactions between proteins in solution.The advantages of our approach are that it allows you to look for weak protein interactions in solution without the need for labels or immobilization. So researchers can observe the proteins of interest and how they interact without worrying that they're disrupting the system they're looking at.

Analysis of the Microfluidics Coupled Mass Photometry Data to Investigate Low-Affinity Protein-Protein Interactions

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Research

JoVE Journal

Biochemistry

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