Molecular approximation between residue 10 of secretin and its receptor demonstrated by photoaffinity labeling

Using photoaffinity labeling, we have previously explored the molecular approximations between multiple positions of secretin and its receptor. Interestingly, the amino-terminal secretin probe incorporating a photolabile residue in position 1 labels the top of the sixth transmembrane domain of the receptor, whereas other probes with photolabile residue in positions 6, 12, 13, 14, 18, 22, and 26 all label the long amino-terminal domain of the secretin receptor. Recently, we have developed a secretin probe that incorporated a radioiodinatable photolabile p-(4-hydroxybenzoyl)phenylalanine in position 10 and demonstrated that it efficiently labeled the secretin receptor in a saturable and specific manner. In this work, we attempted to further map its domain of labeling by cyanogen bromide (CNBr) cleavage of the wild-type and mutant receptors. Surprisingly, this position 10 probe labeled the top of the sixth transmembrane domain of the receptor, a domain labeled by the position 1 probe. These data provide an important constraint for modeling the agonist-bound G protein-coupled secretin receptor and should add substantially to our current understanding of the molecular basis of ligand binding of this important receptor.