CALMODULIN CONTROLS THE ROD PHOTORECEPTOR CNG CHANNEL THROUGH AN UNCONVENTIONAL BINDING-SITE IN THE N-TERMINUS OF THE BETA-SUBUNIT

Calmodulin (CaM) controls the activity of the rod cGMP-gated ion chann el by decreasing the apparent cGMP affinity. We have examined the mech anism of this modulation using electrophysiological and biochemical te chniques. Heteromeric channels, consisting of alpha- and beta-subunits , display a high CaM sensitivity (EC50 less than or equal to 5 nM) sim ilar to the native channel. Using surface plasmon resonance spectrosco py, we identified two unconventional CaM-binding sites (CaM1 and CaM2) , one in each of the N- and the C-terminal regions of the beta-subunit , Ca2+ co-operatively stimulates binding of CaM to these sites exactly within the range of [Ca2+] occurring during a light response. Deletio n of the N-terminal CaM1 site results in channels that are no longer C aM-sensitive, whereas deletion of CaM2 has only minor effects. We disc uss different models to explain the high-affinity binding of CaM.