Figure 3:

Monitoring K⁺ transport by the KdpFABC. (A) The florescent dye DiSC3(5) redistributes between the aqueous phase and the inner leaflet of the vesicle membrane as K⁺ transport out of the vesicle generates a negative electric potential. The resulting increase in dye concentration in the inner leaflet causes fluorescence quenching. Accordingly, when ATP was added to a vesicle suspension, K⁺ transport is reflected by decreasing fluorescence as shown in the lower panel. The initial rate of decrease can be used to quantify transport and the exponential shape reflects inhibition of transport as the potential increases. Experimental data were adopted from Damnjanovic et al. (2013). Lipid vesicles are depicted as grey annuli, Kdp as a multicolored complex and DiSC3(5) molecules as 3 rings. (B) Electric currents are generated by the Kdp complex upon release of a small amount of ATP from caged ATP and can be measured with a black lipid membrane setup with proteoliposomes tightly (and thus capacitively) coupled to the black lipid membrane (BLM). The lower panel shows the time course of the electric current detected by electrodes placed on both sides (cis and trans) of the black lipid membrane. The sign of the transient current (which ceases within a few seconds) indicates transport of positive charge out of the liposomes. Experimental data were adopted from Fendler et al. (1996), Copyright 1996 American Chemical Society. I(t) represents a current meter connected to cis and trans electrodes.