Mechanism of Phototransduction in Retinal Rods and Cones 
channels by having a much higher affinity for 
cGMP (and also adenosine 3':5'-cyclic monophos- 
phate, or cAMP, another important cyclic 
nucleotide) . 
Recently the rod channel in bovine retina was 
purified and cloned by others. On the basis of 
structural homology to the rod channel, Randall 
Reed, another HHMI investigator at Johns Hop- 
kins, has also obtained a cDNA clone for the rat 
olfactory channel. We examined the physiology 
of this channel, after expression in a mammalian 
cell line, and found that it has a current-voltage 
relation remarkably similar to that of the rod 
channel, except that cGMP and cAMP activate 
this channel about 50 times more effectively than 
they do the rod channel. The next step to be taken 
is to generate hybrids between the two channels, 
in order to identify the part of the molecules that 
determines affinities for cyclic nucleotides. 
In addition, with Dr. Reed's help, we have iso- 
lated, again by structural homology, several 
clones from a human retinal cDNA library that 
may encode for both the rod and cone channels 
in the human retina. Experiments are now un- 
der way to test these clones for functional 
expression. 
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