N-Terminal Protein Labeling with N-Hydroxysuccinimide Esters and Microscale Thermophoresis Measurements of Protein-Protein Interactions Using Labeled Protein.

Protein labeling strategies have been explored for decades to study protein structure, function, and regulation. Fluorescent labeling of a protein enables the study of protein-protein interactions through biophysical methods such as microscale thermophoresis (MST). MST measures the directed motion of a fluorescently labeled protein in response to microscopic temperature gradients, and the protein's thermal mobility can be used to determine binding affinity. However, the stoichiometry and site specificity of fluorescent labeling are hard to control, and heterogeneous labeling can generate inaccuracies in binding measurements. Here, we describe an easy-to-apply protocol for high-stoichiometric, site-specific labeling of a protein at its N-terminus with N-hydroxysuccinimide (NHS) esters as a means to measure protein-protein interaction affinity by MST. This protocol includes guidelines for NHS ester labeling, fluorescent-labeled protein purification, and MST measurement using a labeled protein. As an example of the entire workflow, we additionally provide a protocol for labeling a ubiquitin E3 enzyme and testing ubiquitin E2-E3 enzyme binding affinity. These methods are highly adaptable and can be extended for protein interaction studies in various biological and biochemical circumstances.