Regulation of the Rod Photoreceptor Cyclic Nucleotide-Gated Channel

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Abstract
The cGMP-gated channel of rod photoreceptors plays a key role in phototransduction by controlling the flow of cations into the outer segment in response to light-induced changes in cGMP. The channel is a heterotetramer composed of a-subunits required for channel activity and 13-subunits that are important in modulating the activity of channel. Earlier studies have shown that exogenous calmodulin binds to the β-subunit of the channel and modulates the sensitivity of the channel for cGMP in a calcium dependent manner. In addition unidentified Ca2+-dependent endogenous proteins have been reported to modulate the activity of the frog rod channel. In this paper, we investigated whether endogenous calmodulin and other Ca2+binding proteins interact with and modulate the cGMP-gated channel in bovine rod outer segments. Using immunoaffinity techniques in conjunction with ion flux assays, we show that endogenous calmodulin, but not other Ca2+dependent proteins, binds and modulates the rod cGMP-gated channel in bovine rod outer segments. We also show that the β-subunit of the channel is phosphorylated by endogenous and exogenous casein kinase 2. This posttranslational modification, however, does not alter the sensitivity of the channel for cGMP.