Protein Phosphatase 1c Associated with the Cardiac Sodium Calcium Exchanger1 Regulates its Activity by Dephosphorylating Serine 68 Phosphorylated Phospholemman.

The sodium (Na(+))-calcium (Ca(2+)) exchanger 1 (NCX1) is an important regulator of intracellular Ca(2+) homeostasis. Serine 68 phosphorylated phospholemman (pSer-68-PLM) inhibits NCX1 activity. In the context of Na(+)/K(+) ATPase (NKA) regulation, pSer-68-PLM is dephosphorylated by protein phosphatase 1 (PP1). PP1 also associates with NCX1, however, the molecular basis of this association is unknown. In the present study, we aimed to analyze the mechanisms of PP1 targeting to the NCX1-pSer-68-PLM complex and hypothesized that a direct and functional NCX1-PP1 interaction is a prerequisite for pSer-68-PLM dephosphorylation. Using a variety of molecular techniques, we show that PP1 catalytic subunit (PP1c) co-localized, co-fractionated, and co-immunoprecipitated with NCX1 in rat cardiomyocytes, left ventricle lysates and HEK293 cells. Bioinformatic analysis, immunoprecipitations, mutagenesis, pull-down experiments and peptide arrays constrained PP1c anchoring to the K-I/V-F-F motif in the first Ca(2+) binding domain (CBD1) in NCX1. This binding site is also partially in agreement with the extended PP1 binding motif; K-I/V-F-F-X5-8-Φ1Φ2-X8-9-R. The cytosolic loop of NCX1, containing the K-I/V-F-F motif, had no effect on PP1 activity in an in vitro assay. Dephosphorylation of pSer-68-PLM in HEK293 cells was not observed when NCX1 was absent, when the K-I/V-F-F motif was mutated or when the PLM and PP1c binding sites were separated (mimicking calpain cleavage of NCX1). Co-expression of PLM and NCX1 inhibited NCX1 current (both modes). Moreover, co-expression of PLM with NCX1-(F407P) (mutated K-I/V-F-F motif) resulted in NCX1 current being completely abolished. In conclusion, NCX1 is a substrate-specifying PP1c regulator protein, indirectly regulating NCX1 activity through pSer-68-PLM dephosphorylation.