Functional reconstitution of cell-free synthesized purified K_v channels

The study of ion channel activity and the screening of possible inhibitor molecules require reliable methods for production of active channel proteins, their insertion into artificial membranes and for the measurement of their activity. Here we report on cell-free expression of soluble and active K_v1.1 and K_v1.3 channels and their efficient insertion into liposomes. Two complementary methods for the determination of the electrical activity of the proteoliposome-embedded channels were compared using K_v1.1 as a model system: (1) single channel recordings in droplet interface bilayers (DIB) and (2) measurement of the membrane voltage potential generated by a potassium ion diffusion potential using the voltage-sensitive fluorescent dye oxonol VI. Single channel recordings in DIBs proved unreliable because of the non-reproducible fusion of proteoliposomes with an artificial membrane. Therefore, the use of the optical indicator oxonol VI was adapted for 96 well microtiter plates using the ionophore valinomycin as a positive control. The activity of K_v1.1 and K_v1.3 channels was then monitored in the absence and presence of different venom toxins, demonstrating that fluorescent dyes can be used very efficiently when screening small molecules for their channel blocking activity.