ROD AND CONE PHOTORECEPTOR CELLS EXPRESS DISTINCT GENES FOR CGMP-GATED CHANNELS

Signal transduction in vertebrate rod and cone photoreceptor cells inv olves ion channels that are directly gated by the internal messenger c GMP. Rods and each type of cones express genetically related yet diffe rent forms of photopigments. Enzymes that control the light-stimulated hydrolysis of cGMP in rods and cones are also the product of distinct genes. Two different cDNA clones encoding cGMP-gated channels have be en characterized from the chicken retina. Expression of cDNAs in Xenop us oocytes gives rise to cGMP-stimulated channel activity. Antibodies against a synthetic peptide specific for the C-terminal amino acid seq uence derived from one clone stain outer segments of cone but not rod photoreceptors. Therefore chicken rod and cone cells each express diff erent forms of cGMP-gated channels that are genetically related to eac h other. Expression in COS-1 cells produces the complete form of both channel polypeptides, whereas Western blot analysis indicates that cha nnels in outer segment membranes are present in a processed form that is significantly shorter than the full-length polypeptide.