Molecular Cloning and Functional Characterization of a New Modulatory Cyclic Nucleotide-Gated Channel Subunit from Mouse Retina

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Abstract
Cyclic nucleotide-gated (CNG) channels play a key role in olfactory and visual transduction. Native CNG channels are heteromeric complexes consisting of the principal α subunits (CNG1–3), which can form functional channels by themselves, and the modulatory β subunits (CNG4–5). The individual α and β subunits that combine to form the CNG channels in rod photoreceptors (CNG1 + CNG4) and olfactory neurons (CNG2 + CNG4 + CNG5) have been characterized. In contrast, only an α subunit (CNG3) has been identified so far in cone photoreceptors. Here we report the molecular cloning of a new CNG channel subunit (CNG6) from mouse retina. The cDNA of CNG6 encodes a peptide of 694 amino acids with a predicted molecular weight of 80 kDa. Among the CNG channel subunits, CNG6 has the highest overall similarity to the CNG4 β subunit (47% sequence identity). CNG6 transcripts are present in a small subset of retinal photoreceptor cells and also in testis. Heterologous expression of CNG6 in human embryonic kidney 293 cells did not lead to detectable currents. However, when coexpressed with the cone photoreceptor α subunit, CNG6 induced a flickering channel gating, weakened the outward rectification in the presence of extracellular Ca2+, increased the sensitivity forl-cisdiltiazem, and enhanced the cAMP efficacy of the channel. Taken together, the data indicate that CNG6 represents a new CNG channel β subunit that may associate with the CNG3 α subunit to form the native cone channel.