HEAT-STABLE ENTEROTOXIN RECEPTOR GUANYLYL CYCLASE-C IS AN OLIGOMER CONSISTING OF FUNCTIONALLY DISTINCT SUBUNITS, WHICH ARE NONCOVALENTLY LINKED IN THE INTESTINE

Guanylyl cyclase (GC) C is a heat-stable enterotoxin (STa) receptor wi th a monomeric M(r) of approximately 140,000. We calculated from its h ydrodynamic parameters that an active GC-C complex has a M(r) of 393,0 00, suggesting that GC-C is a trimer under native conditions. Both tri meric and dimeric GC-C complexes were detected by I-125-STa binding an d SDS-polyacrylamide gel electrophoresis under non-reducing conditions . The GC activity and STa binding from intestinal brush border membran es comigrated in gel filtration and velocity sedimentation with recomb inant GC-C. However, I-125-STa cross-linking demonstrated that STa rec eptors with molecular masses of 52 and 74 kDa are non-covalently attac hed to GC in the intestine. Radiation inactivation revealed different functional sizes for basal GC activity, STa-stimulated GC activity, an d STa binding (59, 210-240, and 32-52 kDa, respectively). At low radia tion doses, basal GC activity was stimulated, suggesting that GC-C is inhibited by a relatively large, probably internal structure. These re sults suggest that STa may activate GC-C by promoting monomer-monomer interaction (internal ''dimerization'') within a homotrimeric GC-C com plex, and that GC-C is proteolytically modified in the brush border me mbrane but retains its function.