TOPOLOGICAL MIMICRY AND EPITOPE DUPLICATION IN THE GUANYLYL CYCLASE-CRECEPTOR

Guanylyl cyclase C (GCC) is the receptor for the gastrointestinal horm ones, guanylin, and uroguanylin, in addition to the bacterial heat-sta ble enterotoxins, which are one of the major causes of watery diarrhea the world over. GCC is expressed in intestinal cells, colorectal tumo r tissue and tumors originating from metastasis of the colorectal carc inoma. We have earlier generated a monoclonal antibody to human GCC, G CC:B10, which was useful for the immunohistochemical localization of t he receptor in the rat intestine (Nandi A et al., 1997, J Cell Biochem 66:500-511), and identified its epitope to a 63-amino acid stretch in the intracellular domain of GCC. In view of the potential that this a ntibody has for the identification of colorectal tumors, we have chara cterized the epitope for GCC:B10 in this study. Overlapping peptide sy nthesis indicated that the epitope was contained in the sequence HIPPE NIFPLE. This sequence was unique to GCC, and despite a short stretch o f homology with serum amyloid protein and pertussis toxin, no cross re activity was detected. The core epitope was delineated using a random hexameric phage display library, and two categories of sequences were identified, containing either a single, or two adjacent proline residu es. No sequence identified by phage display was identical to the epito pe present in GCC, indicating that phage sequences represented mimotop es of the native epitope. Alignment of these sequences with HIPPENIFPL E suggested duplication of the recognition motif, which was confirmed by peptide synthesis. These studies allowed us not only to define the requirements of epitope recognition by GCC:B10 monoclonal antibody, bu t also to describe a novel means of epitope recognition involving topo logical mimicry and probable duplication of the cognate epitope in the native guanylyl cyclase C receptor sequence.