THE CGMP-GATED CHANNEL OF PHOTORECEPTOR CELLS - ITS STRUCTURAL-PROPERTIES AND ROLE IN PHOTOTRANSDUCTION

The cyclic GMP-gated channel responds to changes in free intracellular cGMP, and as a result, it plays a central role in the phototransducti on process in rod and cone photoreceptor cells. Recent biochemical, im munochemical, and molecular biology studies indicate that this channel consists of a complex of two distinct subunits and one or more associ ated proteins. Primary structural analysis indicates that the alpha an d beta subunits contain a cGMP-binding domain, an even number of membr ane-spanning segments, a voltage sensor motif and a pore region. The l atter two features are found in voltage-gated channels and suggest tha t these two classes of channels have evolved from the same ancestral c hannel protein. A working model for the membrane topography of the cha nnel subunits is proposed based on immunogold labeling studies and seq uence analysis. Recent studies also indicate that calmodulin binds to the 240 kDa protein of the channel complex and modulates the sensitivi ty of the channel for cGMP in a Ca2+-dependent manner. the molecular p roperties of the channel complex and the possible role of Ca2+-calmodu lin modulation of the channel during photoactivation and photorecovery are discussed in relation to the current mechanism of phototransducti on in photoreceptor cells.