The Role of Phospholamban Cysteines in the Activation of the Cardiac Sarcoplasmic Reticulum Calcium Pump by Nitroxyl

Phospholamban (PLN) is an integral membrane protein that regulates the Ca2+ pump (SERCA2a) in cardiac sarcoplasmic reticulum (CSR). Phosphorylation of PLN in response to β-adrenergic stimulation enhances cardiac inotropy by increasing CSR Ca2+ uptake. Nitroxyl (HNO), a new candidate drug therapy for congestive heart failure, improves overall cardiovascular function by increasing Ca2+ release and re-uptake in CSR through a direct interaction with RyR2 and SERCA2a, respectively. Using insect cell ER microsomes expressing SERCA2a +/− PLN (WT and Cys → Ala mutant) we have shown that activation of SERCA2a by HNO is PLN-dependent and entails covalent modification of PLN cysteines. Although HNO stimulates SERCA2a activity by uncoupling PLN from SERCA2a, the role of the cysteine residues in the activation mechanism is not completely understood. We propose that HNO, a thiol oxidant, modifies one or more of the three PLN cysteine residues (C36, C41, C46), affecting the regulatory potency of PLN toward SERCA2a. Examples include intra-molecular disulfide cross-links within single PLN molecules or inter-molecular disulfide cross-links between PLN molecules or PLN and SERCA2a. To test this hypothesis, we have constructed a series of PLN mutants containing single, double and triple cysteine substitutions (alanine replacing cysteine). Each of these mutant PLNs will be co-expressed with SERCA2a in insect cells and cell microsomes will be treated with Angeli's salt (an HNO donor) to determine which cysteine residue(s) are essential for activation monitored by enzyme assay and fluorescence spectroscopy of SERCA2a. The results show that intermolecular PLN disulfides play a minor role in activation by HNO. Studies with the Cys → Ala mutations will be useful in determining which cysteine pairs in PLN contribute to intramolecular disulfide cross-links leading to the relief of PLN inhibition and SERCA2a activation.