Inhibition of PMCA activity by aluminum: Insights into membrane protein regulation through the bilayer

The plasma membrane Ca²⁺-ATPase (PMCA) is essential for cellular calcium homeostasis and is inhibited by aluminum, which displaces Mg²⁺ and traps the enzyme in a phosphorylated state. While aluminum's direct interaction with PMCA is established, how the surrounding lipid environment modulates this inhibition remains unclear. Here, we investigate how amphiphilic assemblies-formed by detergent (C₁₂E₁₀) with either neutral (DMPC) or acidic (brain extract) phospholipids-affect aluminum's impact on PMCA. Using small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS), we show that DMPC-containing systems transition from ellipsoidal micelles to bilayers as the phospholipid fraction increases, while brain extract lipids form elongated mixed micelles without bilayer formation. Aluminum availability, assessed via lumogallion fluorescence, decreases more sharply in the presence of acidic phospholipids, indicating stronger chelation. PMCA activity assays reveal that aluminum inhibition is highly context-dependent. In micellar environments, inhibition dominates as free Al³⁺ directly targets the enzyme. In DMPC bilayers or at high brain extract fractions, inhibition diminishes; in some cases, aluminum even slightly increases PMCA activity. Fluorescent probes (Laurdan and merocyanine-540) indicate that aluminum alters the hydration and packing at the polar headgroup-solvent interface in bilayers but not in micelles. Our results demonstrate that aluminum interacts differently with amphiphilic particles, depending on both the phospholipid polar head and the structure of these lipid mimetic systems. Thus, the effect of aluminum on PMCA is due to direct binding as well as possible indirect modulation through the lipid environment.