Acyl chain differences in phosphatidylethanolamine determine domain formation and LacY distribution in biomimetic model membranes.

Phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) are the two main components of the inner membrane of Escherichia coli. It is well-known that inner membrane contains phospholipids with a nearly constant polar headgroup composition. However, bacteria can regulate the degree of unsaturation of the acyl chains in order to adapt to different external stimuli. Studies on model membranes of mixtures of PE and PG, mimicking the proportions found in E. coli, can provide essential information on the phospholipid organization in biological membranes and may help in the understanding of membrane proteins activity, such as lactose permease (LacY) of E. coli. In this work we have studied how different phosphatidylethanolamines differing in acyl chain saturation influence the formation of laterally segregated domains. Three different phospholipid systems were studied: DOPE:POPG, POPE:POPG, and DPPE:POPG at molar ratios of 3:1. Lipid mixtures were analyzed at 24 and 37 °C through three different model membranes: monolayers, liposomes, and supported lipid bilayers (SLBs). Data from three different techniques, Langmuir isotherms, Laurdan generalized polarization, and atomic force microscopy (AFM), evidenced that only the DPPE:POPG system exhibited coexistence between gel (L(β)) and fluid (L(α)) phases at both 24 and 37 °C . In the POPE:POPG system the L(β)/L(α) coexistence appears at 27 °C. Therefore, in order to investigate the distribution of LacY among phospholipid phases, we have used AFM to explore the distribution of LacY in SLBs of the three phospholipid systems at 27 °C, where the DOPE:POPG is in L(α) phase and POPE:POPG and DPPE:POPG exhibit L(β)/L(α) coexistence. The results demonstrate the preferential insertion of LacY in fluid phase.