Sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA) pump activity is modulated by 
phospholamban (PLB) and sarcolipin (SLN) in cardiac and skeletal muscle. Recent 
data suggest that SLN could play a role in muscle thermogenesis by promoting 
uncoupling of the SERCA pump (Lee, A.G. (2002) Curr. Opin. Struct. Biol. 12, 
547-554 and Bal, N. C., Maurya, S. K., Sopariwala, D. H., Sahoo, S. K., Gupta, 
S. C., Shaikh, S. A., Pant, M., Rowland, L. A., Bombardier, E., Goonasekera, S. 
A., Tupling, A. R., Molkentin, J. D., and Periasamy, M. (2012) Nat. Med. 18, 
1575-1579), but the mechanistic details are unknown. To better define how 
binding of SLN to SERCA promotes uncoupling of SERCA, we compared SLN and SERCA1 
interaction with that of PLB in detail. The homo-bifunctional cross-linker 
(1,6-bismaleimidohexane) was employed to detect dynamic protein interaction 
during the SERCA cycle. Our studies reveal that SLN differs significantly from 
PLB: 1) SLN primarily affects the Vmax of SERCA-mediated Ca(2+) uptake but not 
the pump affinity for Ca(2+); 2) SLN can bind to SERCA in the presence of high 
Ca(2+), but PLB can only interact to the ATP-bound Ca(2+)-free E2 state; and 3) 
unlike PLB, SLN interacts with SERCA throughout the kinetic cycle and promotes 
uncoupling of the SERCA pump. Using SERCA transmembrane mutants, we additionally 
show that PLB and SLN can bind to the same groove but interact with a different 
set of residues on SERCA. These data collectively suggest that SLN is 
functionally distinct from PLB; its ability to interact with SERCA in the 
presence of Ca(2+) causes uncoupling of the SERCA pump and increased heat 
production.