Mapping Interactions Between the Amino-Terminal Region of Secretin and its Receptor using Disulfide-Trapping

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While NMR and crystal structures have defined the molecular basis for docking the carboxyl-terminal region of natural peptide ligands with the disulfide-bonded amino-terminal tail of class B GPCRs, the structural basis for docking the biologically critical amino-terminal regions of these ligands is much less clear. We previously utilized photoaffinity labeling to define spatial approximations between distinct positions within this region of secretin and residues within the core domain of its intact receptor. Now, we use a more powerful disulfide-trapping approach to systematically explore spatial proximities between cysteine residues incorporated into the amino terminus of secretin and into each of the extracellular loops (ECLs) of its receptor. This approach is less disruptive, due to the use of cysteines in place of the large photolabile moieties necessary for photolabeling, and the spatial information is more useful due to the shorter length of the disulfide bonds formed. Cysteines were incorporated into each of the six amino-terminal positions of secretin, with only positions 2 and 5 tolerated to yield reasonable binding affinities and biological activities. These two peptides were used to probe 61 ECL mutants in which cysteine residues were incorporated into the ECLs. The patterns of disulfide formation were quite distinct for the two probes. The position 2 probe predominantly labeled residues in the carboxyl-terminal region of ECL1 and amino-terminal regions of ECL2 and ECL3, while the position 5 probe labeled those within the carboxyl-terminal region of ECL2 and throughout ECL3. These data add substantial new insights for refining our understanding of secretin binding and activation of its receptor.