Domain formation in a fluid mixed lipid bilayer modulated through binding of the C2 protein motif

The role and mechanism of formation of lipid domains in a functional membra ne have generally received limited attention. Our approach, based on the hy pothesis that thermodynamic coupling between lipid-lipid and protein-lipid interactions can lead to domain formation, uses a combination of an experim ental lipid bilayer model system and Monte Carlo computer simulations of a simple model of that system. The experimental system is a fluid bilayer com posed of a binary mixture of phosphatidylcholine (PC) and phosphatidylserin e (PS), containing 4% of a pyrene-labeled anionic phospholipid. Addition of the C2 protein motif (a structural domain found in proteins implicated in eukaryotic signal transduction and cellular trafficking processes) to the b ilayer first increases and then decreases the excimer/monomer ratio of the pyrene fluorescence. We interpret this to mean that protein binding induces anionic lipid domain formation until the anionic lipid becomes saturated w ith protein. Monte Carlo simulations were performed on a lattice representi ng the lipid bilayer to which proteins were added. The important parameters are an unlike lipid-lipid interaction term and an experimentally derived p referential protein-lipid interaction term. The simulations support the exp erimental conclusion and indicate the existence of a maximum in PS domain s ize as a function of protein concentration. Thus, lipid-protein coupling is a possible mechanism for both lipid and protein clustering on a fluid bila yer. Such domains could be precursors of larger lipid-protein clusters ('ra fts'), which could be important in various biological processes such as sig nal transduction at the level of the cell membrane.