Calmodulin controls the rod photoreceptor CNG channel through an unconventional binding site in the N-terminus of the beta-subunit. |
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Authors: | D Weitz M Zoche F Müller M Beyermann H G K?rschen U B Kaupp K W Koch |
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Institution: | Institut für Biologische Informationsverarbeitung, Forschungszentrum Jülich, D-52425 Jülich. |
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Abstract: | Calmodulin (CaM) controls the activity of the rod cGMP-gated ion channel by decreasing the apparent cGMP affinity. We have examined the mechanism of this modulation using electrophysiological and biochemical techniques. Heteromeric channels, consisting of alpha- and beta-subunits, display a high CaM sensitivity (EC50 </=5 nM) similar to the native channel. Using surface plasmon resonance spectroscopy, 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. Deletion of the N-terminal CaM1 site results in channels that are no longer CaM-sensitive, whereas deletion of CaM2 has only minor effects. We discuss different models to explain the high-affinity binding of CaM. |
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