Ca²⁺-dependent desensitization. (A) Current decline in patches expressing either A2-A2-A4-B1b channels (black) or with channels that lack the CNGA4 subunit (A2-A2-A2-B1b; gray) upon exposure to Ca²⁺. The current decline reflects the time course of the desensitization process (see also Fig. S4). Desensitization is fast in A2-A2-A4-B1b channels with endogenous and exogenous calmodulin (black). Channels without CNGA4 desensitize slowly with exogenous calmodulin (left, gray), but rapidly with endogenous calmodulin (right, gray; note the different time scale). (B) Summary of the desensitization kinetics in A2-A2-A4-B1b channels and in channels lacking CNGA4. The time constant of current decline dependent on CNGA4 only in the exogenous complex (channel with CNGA4: τ_fast = 0.7 ± 0.3 s and a_f = 0.68 ± 0.23, seven patches; channel without CNGA4: τ_fast = 1.9 ± 1.4 s and a_f = 0.44 ± 0.16, 10 patches). Desensitization speed in the endogenous complex did not depend on CNGA4 (channels with CNGA4: τ = 0.9 ± 0.5 s, five patches; channels without CNGA4: τ = 0.8 ± 0.3 s, four patches).