Formation of supported lipid bilayer membranes on SiO2 from proteoliposomes containing transmembrane proteins

We report the preparation of protein-containing supported phospholipid bilayers (SPBs) on silica (SiO2). The bilayers are formed from small proteoliposomes, which convert to an SPB when the liposomes adsorb on the surface. The kinetics of this conversion process was followed in real time, using the quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) techniques. The proteoliposomes were prepared by reconstitution of two different proteins into small unilamellar liposomes (diameter similar to 26 nm), creating proteoliposomes with diameters ranging from ca. 50 to 85 nm, depending on protein concentration. The two proteins were proton translocating nicotinamide nucleotide transhydrogenase (TH) from Escherichia coli and gramicidin A (GrA) from Bacillus brevis. The SPB formation process was measured and compared for different protein situations in the liposomes: (i) with the intact TH in the proteoliposomes, (ii) after removal of the water-exposed, hydrophilic domains of TH, and (iii) with GrA-containing proteoliposomes (with no water-soluble domains). In the latter two cases qualitatively similar kinetics were observed as with pure (i.e., without proteins) liposomes. In contrast, the water-exposed hydrophilic domains on TH are found to partially hamper the SPB formation process leaving fractions of nonruptured proteoliposomes on the surface. The latter effect becomes stronger with increasing protein/lipid ratio in the proteoliposomes. A comparison was made between activity measurements of TH-containing proteoliposomes in solution and TH-containing SPBs. The latter results support the conclusions from the QCM-D and SPR measurements.