Abstract
Fluorescence correlation spectroscopy and single-molecule Förster resonance energy transfer are powerful and versatile techniques to quantify and describe molecular interactions. They are particularly well suited to the study of dynamic proteins and assemblies, as they can overcome some of the challenges that stymie more conventional ensemble approaches. In this chapter, we describe the application of these methods to study the interaction of tau with the molecular aggregation inducer, heparin, and the functional binding partner, soluble tubulin. Specifically, we outline the practical aspects of both techniques to characterize the critical first steps of tau aggregation and tau-mediated microtubule polymerization. The information gained from these measurements provides unique insight into tau function and its role in disease.