Atomic force microscopy study of Escherichia coli lactose permease proteolipid sheets

Proteolipid sheets (PLSs) obtained using the vesicle fusion technique on a convenient surface are the base to obtain transmembrane protein biosensors. In this preliminary work, we have screened several physicochemical conditions to optimize the visualization of proteolipid sheets formed between different phospholipid matrices and the membrane protein lactose permease (LacP) by atomic force microscopy (AFM). When LacP was reconstituted in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes, the proteolipid sheets were densely packed with an upper layer that protruded from a background layer. Several lipid protein molar ratios (LPR) were screened. High resolution analysis of the upper layer revealed a quasi-crystalline arrangement formed by small entities that could be attributed to the protein. The approach described here may be suitable for the rational design of biosensors based in other transmembrane proteins.