We have used magnetic resonance to map the interaction surface of an integral 
membrane protein for its regulatory target, an integral membrane enzyme. 
Phospholamban (PLN) regulates cardiac contractility via its modulation of 
sarco(endo)plasmic reticulum calcium ATPase (SERCA) activity. Impairment of this 
regulatory process causes heart failure. To map the molecular details of the 
PLN/SERCA interaction, we have functionally reconstituted SERCA with labeled PLN 
in dodecylphosphocholine micelles for high-resolution NMR spectroscopy and in 
both micelles and lipid bilayers for EPR spectroscopy. Differential 
perturbations in NMR linewidths and chemical shifts, measured as a function of 
position in the PLN sequence, provide a vivid picture of extensive SERCA 
contacts in both cytoplasmic and transmembrane domains of PLN and provide 
structural insight into previously reported functional mutagenesis data. NMR and 
EPR data show clear and complementary evidence for a dynamic (micros-to-ms) 
equilibrium between two conformational states in the cytoplasmic domain of PLN. 
These results support the hypothesis that SERCA attracts the cytoplasmic domain 
of PLN away from the lipid surface, shifting the preexisting equilibrium of PLN 
conformers toward a structure that is poised to interact with the regulatory 
target. EPR shows that this conformational switch behaves similarly in micelles 
and lipid membranes. Based on structural and dynamics data, we propose a model 
in which PLN undergoes allosteric activation upon encountering SERCA.