Understanding of a protein’s function is in many cases gained through an awareness of what it interacts with, with what affinity and at what time. Monitoring protein-protein interactions (PPIs) therefore can provide deep insights into everything from basic biology to the mechanisms of disease.
Limitations of existing systems for studying PPIs
There are a wide array of technologies used for studying PPIs - perhaps the two most widely used are Isothermal Calorimetry (ITC) and Surface Plasmon Resonance (SPR).
ITC is a true in-solution technique, in which the heat either released or absorbed in molecular binding during gradual titration is monitored. However, ITC generally requires large amounts of protein sample (typically 300-2000 µL at 10-50 µM concentration), and is dependent on the interaction of interest having a sufficiently large enthalpic change, which is not always the case.
SPR requires less material than ITC, but is not truly in solution, as it requires one of the protein binding partners to be tethered to a chip surface to monitor interactions through changes in plasmon resonance. Whilst highly sensitive, the requirement for surface functionalization means the analysis is not in solution. Consequently, surface attachment of proteins in the right orientation can be extremely challenging and in some cases lead to immobilisation artefacts and, where diffusion is slower than association, mass transport effects can occur.
The ideal therefore would be ITC quality in-solution data with SPR amounts of material. At Fluidic Analytics, this is what we’re working towards.
Using changes in hydrodynamic radius to study PPIs
The sizing of proteins using microfluidic diffusional sizing enables detection of PPIs in simple mixtures using latent labelling chemistry. This was initially demonstrated by Yates et al, using sizing to monitor a Parkinson’s-related immune complex formed by a nanobody and equimolar (5 μM) α-synuclein. This approach has since been developed into the technology that underpins the Fluidity One.
If fluorescently labelled samples are available, PPIs can be monitored in complex mixtures such as cell lysates (a; Arosio 2015, Zhang 2016).
In this case the change in size of the nanobody in the presence of α-synuclein allows the detection of the binding not only in homogeneous solution but also in cell lysate (b). Further to this, the nature of the interacting protein can be assessed, as is seen in the difference in size between nanobody bound to monomeric synuclein, and synuclein fibrils (d). The ability to measure the hydrodynamic radius of fluorescent proteins and biomolecules will be an enabling feature of the Fluidity One-W.
Critically all of these measurements are performed using proteins in solution in their native state, without the use of a matrix or surface, with high sensitivity and little volume. Ultimately this approach will enable kinetics and thermodynamics of binding reactions to be determined.