Phospholamban Interacts with HAX-1, a Mitochondrial Protein with Anti-apoptotic Function

Phospholamban (PLN) is a key regulator of Ca2+ homeostasis and contractility in the heart. Its regulatory effects are mediated through its interaction with the sarcoplasmic reticulum Ca2+-ATPase, (SERCA2a), resulting in alterations of its Ca2+-affinity. To identify additional proteins that may interact with PLN, we used the yeast-two-hybrid system to screen an adult human cardiac cDNA library. HS-1 associated protein X-1 (HAX-1) was identified as a PLN-binding partner. The minimal binding regions were mapped to amino acid residues 203–245 for HAX-1 and residues 16–22 for PLN. The interaction between the two proteins was confirmed using GST-HAX-1, bound to the glutathione-matrix, which specifically adsorbed native PLN from human or mouse cardiac homogenates, while in reciprocal binding studies, recombinant His-HAX-1 bound GST-PLN. Kinetic studies using surface plasmon resonance yielded a KD of ∼ 1 μM as the binding affinity for the PLN/HAX-1 complex. Phosphorylation of PLN by cAMP-dependent protein kinase reduced binding to HAX-1, while increasing concentrations of Ca2+ diminished the PLN/HAX-1 interaction in a dose-dependent manner. HAX-1 concentrated to mitochondria, but upon transient co-transfection of HEK 293 cells with PLN, HAX-1 redistributed and co-localized with PLN at the endoplasmic reticulum. Analysis of the anti-apoptotic function of HAX-1 revealed that the presence of PLN enhanced the HAX-1 protective effects from hypoxia/reoxygenation-induced cell death. These findings suggest a possible link between the Ca2+ handling by the sarcoplasmic reticulum and cell survival mediated by the PLN/HAX-1 interaction.