Myosin Binding to Human Cardiac Thin Filaments Containing Tropomyosin Carrying DCM & HCM Mutations; Fitting of Complex Binding Transients

Abstract

Myosin S1 binds to pyrene labelled thin filaments in a calcium dependent manner. When mixing an excess of S1 with thin filaments at low calcium the transient is complex in form and requires a complete model of the regulated binding to fit the data. Using the recently published Monte Carlo version of the Mckillop & Geeves model (REF) we have now fitted a complete set of calcium dependent myosin binding transients to thin filaments containing WT human tropomyosin with human cardiac troponin. In addition we have analysed the data for 5 tropomyosin mutations carrying HCM (E175N, & E180G) or DCM mutations (E54K , E40K and D230N) in one or both Tm chains of the dimer. All transients can be well described by the model and for each case the calcium dependent data can be described by a single set of parameters with a change only in the apparent value and calcium dependence of KB (the equilibrium constant between the blocked and closed states of the filament) small secondary effects may be present in value of KT ( equilibrium between closed and open states). For the HCM mutations the value of KB showed enhanced calcium sensitivity with little change in the value of KB at high and low calcium. In contrast, preliminary data for DCM mutations showed little change in the calcium dependence of KB, but larger changes in the low calcium value of KB. This could mean that the DCM mutations turn off the thin filament more effectively. All other constants remain unchanged within experimental error.Supported by the Welcome Trust, NIH R01 AR048776 and Serbian Ministry of Science grants III41007 and OI174028.