Mechanism of PhototransdiicHon in Retinal Rods and Cones 
mediating olfactory transduction in olfactory 
cilia. These channels show both similarities and 
differences. In particular, the rod and olfactory 
channels have similar ion-permeation character- 
istics, but the olfactory channel shows a 30-fold 
higher affinity for cyclic nucleotides. To under- 
stand the molecular determinant for this differ- 
ence, we, in collaboration with Randall Reed 
(HHMI, Johns Hopkins University), have con- 
structed chimeras between the rod and the olfac- 
tory channels and tested for modifiications in 
function. The difference in amino acid residues at 
the putative cyclic nucleotide-binding site on 
the two channel molecules has little to do with 
their difference in affinity for cyclic nucleotides. 
Other regions of the molecule must therefore be 
involved, possibly through steric or allosteric in- 
teractions in the folded molecule. Identification 
of these regions is still in progress. 
Separately, several independent clones have 
been isolated from a human retinal cDNA library 
based on structural homology to the published 
sequence of the bovine rod channel. One of these 
has been successfully expressed in human 293 
cells and found to have properties identical to the 
bovine rod channel. Thus this clone most proba- 
bly encodes for the human rod channel. The 
other clones have yet to be expressed function- 
ally in a cell line. Based on the presence of a puta- 
tive cyclic nucleotide-binding domain as well as 
other features in their nucleotide sequences, 
however, they probably also encode for cyclic 
nucleotide-gated channels. Antipeptide antibod- 
ies are being made to identify the locations of the 
encoded proteins in the retina. Continuing ef- 
forts are also being made to express these clones 
functionally. Cloning at the genomic level is be- 
ing carried out to provide further clarification. 
Part of the above work is supported by a grant 
from the National Institutes of Health. 
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