Gating of Heteromeric Retinal Rod Channels by Cyclic AMP: Role of the C-Terminal and Pore Domains

AbstractCyclic nucleotide-gated channels are tetramers composed of homologous α and β subunits. C-terminal truncation mutants of the α and β subunits of the retinal rod channel were expressed in Xenopus oocytes, and analyzed for cGMP- and cAMP-induced currents (single-channel records and macroscopic currents). When the α subunit truncated downstream of the cGMP-binding site (αD608stop) is co-injected with truncated β subunits, the heteromeric channels present a drastic increase of cAMP sensitivity. A partial effect is observed with heteromeric αR656stop-containing channels, while αK665stop-containing channels behave like αwt/βwt. The three truncated α subunits have wild-type activity when expressed alone. Heteromeric channels composed of αwt or truncated α subunits and chimeric β subunits containing the pore domain of the α subunit have the same cAMP sensitivity as α-only channels. The results disclose the key role of two domains distinct from the nucleotide binding site in the gating of heteromeric channels by cAMP: the pore of the β subunit, which has an activating effect, and a conserved domain situated downstream of the cGMP-binding site in the α subunit (I609-K665), which inhibits this effect