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Original Article

^a Center for Neurobiology and Behavior, Howard Hughes Medical Institute, Columbia University, New York, NY 10032
^b Department of Pharmacology, Howard Hughes Medical Institute, Columbia University, New York, NY 10032
Center for Neurobiology and Behavior, Howard Hughes Medical Institute, Columbia University, 722 West 168 Street, New York, NY 10032.(212) 795-7997

ecy4{at}columbia.edu

CNG channels in vivo are heteromers of homologous and β subunits that each contain a six-transmembrane segment domain and a COOH-terminal cytoplasmic cyclic nucleotide binding domain (BD). In heterologous expression systems, heteromeric β channels activate with greater sensitivity to ligand than do homomeric channels; however, ligand-gating of channels containing only β subunit BDs has never been studied because β subunits cannot form functional homomeric CNG channels. To characterize directly the contribution of the β subunit BD to ligand-gating, we constructed a chimeric subunit, X-β, whose BD sequence was that of the β subunit CNG5 from rat, but whose sequence outside the BD was derived from subunits. For comparison, we constructed another chimera, X-, whose sequence outside the BD was identical to that of X-β, but whose BD sequence was that of the subunit CNG2 from catfish. When expressed in Xenopus oocytes, X-β and X- each formed functional homomeric channels activated by both cAMP and cGMP. This is the first demonstration that the β subunit BD can couple ligand binding to activation in the absence of subunit BD residues. Notably, both agonists activate X-β more effectively than X- (higher opening efficacy and lower K_1/2). The BD is believed to comprise two functionally distinct subdomains: (1) the roll subdomain (β-roll and flanking A- and B-helices) and (2) the C-helix subdomain. Opening efficacy was previously believed to be controlled primarily by the C-helix, but when we made additional chimeras by exchanging the subdomains between X-β and X-, we found that both subdomains contain significant determinants of efficacy and agonist selectivity. In particular, only channels containing the roll subdomain of the β subunit had high efficacy. Thermodynamic linkage analysis shows that interaction between the two subdomains accounts for a significant portion of their contribution to activation energetics.

Key Words: cAMP • cGMP • ligand-gated channels • allosteric activation • thermodynamic linkage

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