Interestingly, the transport activity of the most active BU 1 remained unaffected by the presence of cholesterol in the membrane. Therefore, we were curious if changes in the membrane thickness could influence the transport activity of BU 1. The transport rates of 1 in LUVs made of phosphocholines (PCs) with two monounsaturated fatty acids with varied chain lengths were measured. If the exchange of anions at the membrane periphery would be the rate-limiting step of the transport process, the transport rates should be similar regardless of the membrane thickness. On the other hand, a decrease in the transport activity with increasing length of the PC acyl chains would suggest that the diffusion of 1 is the rate-determining step of the transport process.