This proposal focuses on the structural differences between the two homologous proteins, troponin-C and calmodulin, and the interactions of these two proteins with their respective target molecules: skeletal troponin-I and skeletal myosin light chain kinase. The specific aims are to: (1) Study the spatial relation of the bound ions in troponin-C and calmodulin, and determine the effects on the arrangement owing to binding of metal and target proteins. (2) Compare the static and kinetic aspects of binding of the two proteins to troponin-I. (3) Characterize the interactions between calmodulin and myosin light chain kinase, and compare troponin-C with calmodulin in such interactions. (4) Identify the regions of these proteins involved in complex formation. A number of physical and chemical techniques will be used, including a) fluorescence energy transfer, b) lanthanide luminescence, c) rapid mixing, d) chemical modification, and e) electron paramagnetic resonance. Information obtained from these studies will allow us to better understand the molecular mechanism of the regulatory actions for both proteins. Such information may be helpful towards the cure of certain disorders in muscle and non-muscle systems.