Sarcolipin is a Conformation-Dependent Regulator of the Sarcoplasmic Reticulum Calcium Pump SERCA

Muscle relaxation is enabled by the sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA), which removes calcium from the cytosol and returns it to the lumen of the sarcoplasmic reticulum. During transport, SERCA adopts a variety of conformational states which differ in their structure and affinity for substrates. The transition between these states is mediated by calcium and ATP binding and the formation of an aspartyl-phosphate intermediate, which enables the transport of calcium across the membrane. SERCA function is highly regulated because of the importance of calcium in processes such as muscle contraction-relaxation. A family of tissue-specific transmembrane regulatory subunits interact with SERCA, exemplified by sarcolipin (SLN) in skeletal muscle and phospholamban (PLN) in cardiac muscle. SLN and PLN are known to alter the apparent calcium affinity and maximal activity of SERCA. In the present study, we investigated SLN inhibition of SERCA under conditions that varied the substrate-dependent conformational state of SERCA. Measuring both calcium-dependent ATP hydrolysis and charge translocation, we found that SLN inhibition was dependent on the initial state of SERCA. Under substrate conditions that poised SERCA in the calcium-free E2 state, SLN was more inhibitory and impacted both the maximal activity and apparent calcium affinity of SERCA. In contrast, SLN inhibition was reduced under pre-incubation conditions that favored the calcium-bound E1 state of SERCA. We conclude that SLN is capable of distinct modes of interaction with SERCA depending on the conformational state, and that the mode of interaction exhibits conformational memory in that the initial state persists during steady-state turnover of SERCA.