MOLECULAR MECHANISM FOR LIGAND DISCRIMINATION OF CYCLIC NUCLEOTIDE-GATED CHANNELS

Cyclic nucleotide-gated ion channels of retinal photoreceptors and olf actory neurons are differentially activated by ligands that vary only in their purine ring structure. The nucleotide selectivity of the bovi ne rod cyclic nucleotide-gated channel (cGMP > cIMP much greater than cAMP) was significantly altered by neutralization of a single aspartic acid residue in the binding domain (cGMP greater than or equal to cAM P > cIMP), Substitution by a nonpolar residue at this position inverte d agonist selectivity (cAMP much greater than cIMP greater than or equ al to cGMP), These effects resulted from an alteration in the relative ability of the agonists to promote the allosteric conformational chan ge associated with channel activation, not from a modification in thei r initial binding affinity. We propose a general mechanism for guanine nucleotide discrimination, in common with that observed in high affin ity GTP-binding proteins, involving the formation of a pair of hydroge n bonds between the aspartic acid side chain and N1 and N2 of the guan ine ring.