Nanomolar ATP binding to single myosin cross-bridges in rigor: a molecular approach to studying myosin ATP kinetics using single human cardiomyocytes.

Our knowledge in the field of cardiac muscle and associated cardiomyopathies has been evolving incrementally over the past 60 years and all was possible due to the parallel progress in techniques and methods allowing to take a fresh glimpse at an old problem. Here, we describe an exciting tool used to examine the various states of the human cardiac myosin at the single molecule level. By imaging single Alexa(647)-ATP binding to permeabilised cardiomyocytes using total internal reflection fluorescence (TIRF) microscopy, we are able to acquire large populations of events in a short timeframe (~ 5000 sites in ~ 10 min) and measure each binding event with high spatio-temporal resolution. The applied algorithm decomposes the point pattern of single molecule binding events into individually distinct binding sites that enables us to recover kinetic parameters, such as bound or free time per site. This single molecule binding approach is a useful tool used to examine muscle contractility. Of particular importance is its application to probing the dynamic lifetimes and proportion of myosins in the super-relaxed state in human cardiomyopathies.