Sarcolipin interaction with SERCA is distinct from Phospholamban; only Sarcolipin can promote uncoupling of the SERCA pump

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) Current opinion in structural biology 12(4):547-554 and Bal et al (2012) Nature Medicine 18(10):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 homobifunctional cross-linker (1, 6bismaleimidohexane (BMH)) 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 uptake but not the pump affinity for Ca 2) SLN can bind to SERCA in the presence of high Ca but PLB can only interact to the ATP bound Ca 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 causes uncoupling of the SERCA pump and increased heat production.