Role of phospholemman phosphorylation sites in mediating kinase-dependent regulation of the Na⁺-K⁺-ATPase

Phospholemman (PLM) is a major target for phosphorylation mediated by both PKA (at Ser68) and PKC (at both Ser63 and Ser68) in the heart. In intact cardiac myocytes, PLM associates with and inhibits Na⁺-K⁺-ATPase (NKA), mainly by reducing its affinity for internal Na⁺. The inhibition is relieved upon PLM phosphorylation by PKA or PKC. The aim here was to distinguish the role of the Ser63 and Ser68 PLM phosphorylation sites in mediating kinase-induced modulation of NKA function. We expressed wild-type (WT) PLM and S63A, S68A, and AA (Ser63 and Ser68 to alanine double mutant) PLM mutants in HeLa cells that stably express rat NKA-α₁ and we measured the effect of PKA and PKC activation on NKA-mediated intracellular Na⁺ concentration decline. PLM expression (WT or mutant) significantly decreased the apparent NKA affinity for internal Na⁺ and had no significant effect on the maximum pump rate (V_max). PKA activation with forskolin (20 μM) restored NKA Na⁺ affinity in cells expressing WT but not AA PLM and did not affect V_max in either case. Similarly, PKC activation with 300 nM phorbol 12,13-dibutyrate increased NKA Na⁺ affinity in cells expressing WT, S63A, and S68A PLM and had no effect in cells expressing AA PLM. Neither forskolin nor phorbol 12,13-dibutyrate affected NKA function in the absence of PLM. We conclude that PLM phosphorylation at either Ser63 or Ser68 is both necessary and sufficient for completely relieving the PLM-induced NKA inhibition.