Activating and deactivating roles of lipid bilayers on the Ca ²⁺-ATPase/phospholamban complex

The physicochemical properties of the lipid bilayer shape the structure and topology of membrane proteins and regulate their biological function. Here, we investigated the functional effects of various lipid bilayer compositions on the sarcoplasmic reticulum (SR) Ca ²⁺-ATPase (SERCA) in the presence and absence of its endogenous regulator, phospholamban (PLN). In the cardiac muscle, SERCA hydrolyzes one ATP molecule to translocate two Ca ²⁺ ions into the SR membrane per enzymatic cycle. Unphosphorylated PLN reduces SERCA's affinity for Ca ²⁺ and affects the enzymatic turnover. We varied bilayer thickness, headgroup, and fluidity and found that both the maximal velocity (V _max) of the enzyme and its apparent affinity for Ca ²⁺ (K _Ca) are strongly affected. Our results show that (a) SERCA's V _max has a biphasic dependence on bilayer thickness, reaching maximum activity with 22-carbon lipid chain length, (b) phosphatidylethanolamine (PE) and phosphatidylserine (PS) increase Ca ²⁺ affinity, and (c) monounsaturated lipids afford higher SERCA V _max and Ca ²⁺ affinity than diunsaturated lipids. The presence of PLN removes the activating effect of PE and shifts SERCA's activity profile, with a maximal activity reached in bilayers with 20-carbon lipid chain length. Our results in synthetic lipid systems compare well with those carried out in native SR lipids. Importantly, we found that specific membrane compositions closely reproduce PLN effects (V _max and K _Ca) found in living cells, reconciling an ongoing controversy regarding the regulatory role of PLN on SERCA function. Taken with the physiological changes occurring in the SR membrane composition, these studies underscore a possible allosteric role of the lipid bilayers on the SERCA/PLN complex.