Phosphorylation by protein kinase A and dephosphorylation by protein phosphatase 
1 modulate the inhibitory activity of phospholamban (PLN), the endogenous 
regulator of the sarco(endo)plasmic reticulum calcium Ca(2+) ATPase (SERCA). 
This cyclic mechanism constitutes the driving force for calcium reuptake from 
the cytoplasm into the myocite lumen, regulating cardiac contractility. PLN 
undergoes a conformational transition between a relaxed (R) and tense (T) state, 
an equilibrium perturbed by the addition of SERCA. Here, we show that the single 
phosphoryl transfer at Ser16 induces a more pronounced conformational switch to 
the R state in phosphorylated PLN (pPLN). The binding affinity of PLN to SERCA 
is not affected (K(d) values for the transmembrane domains of pPLN and PLN are 
approximately 60 microM), supporting the hypothesis that phosphorylation at 
Ser16 does not dissociate PLN from SERCA. However, the binding surface and 
dynamics in domain Ib (residues 22-31) change substantially upon 
phosphorylation. Since PLN can be singly or doubly phosphorylated at Ser16 and 
Thr17, we propose that these sites remotely control the conformation of domain 
Ib. These findings constitute a paradigm for how post-translational 
modifications such as phosphorylation in the cytoplasmic portion of membrane 
proteins control intramembrane protein-protein interactions.