Whole-cell currents through ChR2, β1-ChR2, and β1-ChR2-ChR2 channels. (A) Linkage of the C-terminus of the β1 subunit to the N-terminus of ChR2 resulted in typical light-induced inward currents in the voltage-clamp mode. The plateau current relative to the transient current was similar for all three channels (52.9 ± 7.6% in ChR2, 42.8 ± 8.2% in β1-ChR2, and 42.0 ± 10.8% in β1-ChR2-ChR2). (B) Currents were significantly reduced when injecting 10 ng cRNA per oocyte. Notably, coupling of a second ChR2 did not alter expression (possible reasons are discussed in the text). Number of measurements were n = 27 (ChR2), n = 20 (β1-ChR2), and n = 21 (β1-ChR2-ChR2; *, P < 0.05 versus ChR2). Error bars represent SEM.