Figure 4.

Phosphorylation of CNGA1 but not CNGB1 prevents inhibition of rod CNG channels by Ca²⁺/CaM. The percentage increase in cGMP K_1/2 values by Ca²⁺/CaM was calculated for each channel type based on the results in Table 1. Ca²⁺/CaM had little effect on channels containing wild-type CNGA1 subunits (CNGA1/CNGB1 and CNGA1/CNGB1_Y1097F) when conditions promoted phosphorylation but greatly increased the K_1/2 for cGMP activation when conditions promoted dephosphorylation. In contrast, Ca²⁺/CaM had similar effects on channels containing CNGA1_Y498F channels (CNGA1_Y498F/CNGB1 and CNGA1_Y498F/CNGB1_Y1097F) when either phosphorylation or dephosphorylation were promoted. Note that there was little difference in the phosphorylation versus dephosphorylation dependence of Ca²⁺/CaM modulation in channels in which the CNGB1 subunit contains the only phosphorylation site (CNGA1_Y498F/CNGB1).