Direct Evaluation of Protein-Lipid Contacts Reveals Protein Membrane Immersion and Isotropic Bicelle Structure

The solvation of membrane proteins by both lipids and water makes their membrane immersion difficult to predict and the choice of a membrane mimic challenging. To characterize protein-lipid contacts and bicelle membrane mimics, we examined protein-lipid cross-relaxation of integrin αIIb and β3(A711P) transmembrane helices in isotropic phospholipid bicelles (q = 0.5 and 0.7). Long-chain bicelle lipids dominated contacts with central helix segments, whereas both short- and long-chain lipids contacted the terminal turns of each helix in corroboration of the mixed bicelle model. The saturation transfer profiles from long-chain lipids directly established helix midpoints in the lipid bilayer. Lipid headgroups and water molecules engaged the side chains of buried serine and threonine in competition with intrahelical hydrogen bonding, illustrating that polar side chains seek the most favorable electrostatic contacts.