Calmodulin binding to CNGB1b. (A) Ca²⁺-dependent binding of calmodulin to an LQ peptide representing the N-terminal calmodulin-binding site of CNGB1b. The peptide binds with a $K1/2Ca$ of 12 nM to wild-type calmodulin (CaM^wt, •), with $K1/2Ca$ = 21 nM to the calmodulin mutant CaM¹² (▴), and with $K1/2Ca$ = 27 nM to the mutant CaM³⁴ (▾). The peptide produced neither a Ca²⁺-dependent fluorescence signal by itself (▪) nor in the presence of the inactive calmodulin mutant CaM¹²³⁴ (○). (B) An L→E exchange that corresponds to the key position 183 of the CNGB1b subunit eliminates binding of the peptide to calmodulin with one disabled lobe. The EQ peptide binds wild-type calmodulin with a $K1/2Ca$ of 60 nM (CaM^wt, •). However, no binding was observed with the calmodulin mutants CaM¹² (▴) and CaM³⁴ (▾), illustrating that the LQ site is necessary for the Ca²⁺-dependent association of the CNGB1b peptide with partially liganded calmodulin.