Antibodies against synthetic peptides used to determine the topology and site of glycosylation of the cGMP-gated channel from bovine rod photoreceptors

Peptides corresponding to amino acids 321-339 (peptide GS21) and 416-431 (peptide GS31) of the cGMP-gated channel from bovine rod photoreceptors were synthesized and used as antigens for the preparation of polyclonal antibodies. After affinity purification, both antipeptide antibodies were found to bind specifically to the channel protein after Western blotting, but only the antibody against GS21 gave satisfactory results on enzyme-linked immunosorbent assay and electron microscopy. Using immunocytochemistry, we were able to localize amino acids 321-339 to the extracellular side of the rod photoreceptor plasma membrane. By synthesizing heptapeptides corresponding to amino acids 324-330 (peptide GS2s) and 420-426 (peptide GS3s), we were able to affinity purify antibodies specific for two N-glycosylation consensus sites in the channel protein. As assessed by Western blotting, antibodies against GS3s were found to bind to both the glycosylated and deglycosylated channel proteins, whereas antibodies against GS2s only bound to the channel protein after enzymatic deglycosylation. Together, these results allow the refinement of folding models for the cGMP-gated channel and implicate Asn-327 as being the sole site of N-glycosylation