Solubilization and functional reconstitution of the cGMP-dependent cation channel from bovine rod outer segments

The protein(s) that constitute(s) the cGMP-regulated channel in vertebrate photoreceptors has been solubilized from rod outer segment membranes and reincorporated into the membrane of calcium-containing liposomes. The properties of the reconstituted channel protein were determined by studying the cGMP-stimulated efflux of Ca2+ from these liposomes. Among several detergents tested the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) proved to be the most suitable. Solubilization of channel activity was found to be optimal at a detergent concentration of about 18 mM. The presence of Ca2+ ions and phospholipids during solubilization greatly increased the channel stability. The reconstituted channel shared most but not all properties with the channel in situ. It is cooperatively activated by cGMP with an EC50 of 19 microM. The cooperativity as determined from Hill plots was n = 2.7. Unlike the cGMP-sensitive channel in the native membrane of isolated discs and excised patches of plasma membrane it is not blocked by l-cis-diltiazem. Reconstitution of this channel protein(s) may serve as a valuable tool for identifying the polypeptide composition and to study structural and functional aspects of the purified protein(s).