Investigating Structural Changes in the Lipid Bilayer upon Insertion of the Transmembrane Domain of the Membrane-Bound Protein Phospholamban Utilizing ³¹P and ²H Solid-State NMR Spectroscopy

Phospholamban (PLB) is a 52-amino acid integral membrane protein that regulates the flow of Ca²⁺ ions in cardiac muscle cells. In the present study, the transmembrane domain of PLB (24-52) was incorporated into phospholipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC). Solid-state ³¹P and ²H NMR experiments were carried out to study the behavior of POPC bilayers in the presence of the hydrophobic peptide PLB at temperatures ranging from 30°C to 60°C. The PLB peptide concentration varied from 0 mol % to 6 mol % with respect to POPC. Solid-state ³¹P NMR spectroscopy is a valuable technique to study the different phases formed by phospholipid membranes. ³¹P NMR results suggest that the transmembrane protein phospholamban is incorporated successfully into the bilayer and the effects are observed in the lipid lamellar phase. Simulations of the ³¹P NMR spectra were carried out to reveal the formation of different vesicle sizes upon PLB insertion. The bilayer vesicles fragmented into smaller sizes by increasing the concentration of PLB with respect to POPC. Finally, molecular order parameters (S_CD) were calculated by performing ²H solid-state NMR studies on deuterated POPC (sn-1 chain) phospholipid bilayers when the PLB peptide was inserted into the membrane.