The C-linker of TAX-4 confers high apparent affinity for cAMP and cGMP. Channel notations as in Fig. . The thick gray line in ROPTL2 icon indicates the pore domain of OLF (see text). (A) Ratio of the maximal currents elicited by saturating concentrations of cAMP and cGMP for RET:TAX-4 chimeras. cGMP was applied at concentrations ≥30 μM for RTL2, ROPTL2, RTL, RTC, and TAX-4 and ≥300 μM for RET, RTB, and RTL1. cAMP was applied at concentrations ≥3 mM for RTL, RTC, and TAX-4 and at 10 mM for all other constructs. Currents were measured at −80 or −100 mV. The number of patches for each channel are in parentheses. Error bars show standard errors. The mean values for the ratios are as follows (±SEM): 0.015 ± 0.002 (RET), 0.0092 ± 0.001 (RTB), 0.042 ± 0.003 (RTL1), 0.816 ± 0.03 (RTL2), 0.79 ± 0.03 (ROPTL2), 0.98 ± 0.004 (RTL), 0.80 ± 0.03 (RTC), and 0.99 ± 0.007 (TAX-4). (B) *K*_{1/2} for cGMP. Mean values are: 45 μM (*n* = 9) for RET, 63 μM (*n* = 5) for RTB, 24 μM (*n* = 4) for RTL1, 4 μM (*n* = 4) for RTL2, 3 μM for ROPTL2 (*n* = 4), 1.0 μM (*n* = 8) for RTL, 1.3 μM (*n* = 8) for RTC, and 0.4 μM for TAX-4 (*n* = 6). (C) *K*_{1/2} for cAMP. Mean values are: 2,140 μM (*n* = 7) for RET, 2,050 μM (*n* = 2) for RTB, 2,260 μM (*n* = 2) for RTL1, 1,035 μM (*n* = 3) for RTL2, 990 μM for ROPTL2 (*n* = 5), 213 μM (*n* = 6) for RTL, 400 μM (*n* = 4) for RTC, and 76 μM for TAX-4 (*n* = 3). Note that (a) an increased cAMP/cGMP maximal current ratio is paralleled by an increased apparent affinity for both cAMP and cGMP, and (b) higher sensitivity to cGMP than cAMP is conserved in all chimeras.

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