Mechanisms for cardiac calcium pump activation by its substrate and a synthetic allosteric modulator using fluorescence lifetime imaging

The discovery of allosteric modulators is an emerging paradigm in drug discovery, and signal transduction is a subtle and dynamic process that is challenging to characterize. We developed a time-correlated single photon-counting imaging approach to investigate the structural mechanisms for small-molecule activation of the cardiac sarcoplasmic reticulum CA²⁺-ATPase, a pharmacologically important pump that transports CA²⁺ at the expense of adenosine triphosphate (ATP) hydrolysis. We first tested whether the dissociation of sarcoplasmic reticulum CA²⁺-ATPase from its regulatory protein phospholamban is required for small-molecule activation. We found that CDN1163, a validated sarcoplasmic reticulum CA²⁺-ATPase activator, does not have significant effects on the stability of the sarcoplasmic reticulum CA²⁺-ATPase-phospholamban complex. Time-correlated single photon-counting imaging experiments using the nonhydrolyzable ATP analog β,γ-Methyleneadenosine 5'-triphosphate (AMP-PCP) showed ATP is an allosteric modulator of sarcoplasmic reticulum CA²⁺-ATPase, increasing the fraction of catalytically competent structures at physiologically relevant CA²⁺ concentrations. Unlike ATP, CDN1163 alone has no significant effects on the CA²⁺-dependent shifts in the structural populations of sarcoplasmic reticulum CA²⁺-ATPase, and it does not increase the pump's affinity for CA²⁺ ions. However, we found that CDN1163 enhances the ATP-mediated modulatory effects to increase the population of catalytically competent sarcoplasmic reticulum CA²⁺-ATPase structures. Importantly, this structural shift occurs within the physiological window of CA²⁺ concentrations at which sarcoplasmic reticulum CA²⁺-ATPase operates. We demonstrated that ATP is both a substrate and modulator of sarcoplasmic reticulum CA²⁺-ATPase and showed that CDN1163 and ATP act synergistically to populate sarcoplasmic reticulum CA²⁺-ATPase structures that are primed for phosphorylation. This study provides novel insights into the structural mechanisms for sarcoplasmic reticulum CA²⁺-ATPase activation by its substrate and a synthetic allosteric modulator.