Sarcolipin uncouples hydrolysis of ATP from accumulation of Ca2+ by the Ca2+-ATPase of skeletal-muscle sarcoplasmic reticulum

Sarcolipin (SLN) is a small peptide found in the sarcoplasmic reticulum of skeletal muscle. It is predicted to contain a single hydrophobic transmembrane alpha-helix. Fluorescence emission spectra for the single Trp residue of SLN suggest that SLN incorporates fully into bilayers of dioleoylphosphatidylcholine, but only partially into bilayers of phosphatidylcholines with long (C-22 or C-24) fatty acyl chains. The fluorescence of SLN is quenched in bilayers of dibroinostearoylphosphatidylcholine, also consistent with incorporation into the lipid bilayer. SLN was reconstituted with the Ca2+-ATPase of skeletal-muscle sarcoplasmic reticulum. Even at a 50:1 molar ratio of SLN/ ATPase, SLN had no significant effect on the rate of ATP hydrolysis by the ATPase or on the Ca2+-dependence of ATP hydrolysis. However, at a molar ratio of SLN/ATPase of 2:1 or higher the presence of SLN resulted in a marked decrease in the level of accumulation of Ca2+ by reconstituted vesicles. The effect of SLN was structurally specific and did not result from a breakdown in the vesicular structure or from the formation of non-specific ion channels. Vesicles were impermeable to Ca2+ in the absence of ATP in the external medium. The effects of SLN on accumulation of Ca2+ can be simulated assuming that SLN increases the rate of slippage on the ATPase and the rate of passive leak of Ca2+ mediated by the ATPase. It is suggested that the presence of SLN could be important in non-shivering thermogenesis, a process in which heat is generated by hydrolysis of ATP by skeletal-muscle sarcoplasmic reticulum.