Guanylyl cyclase C is an N-linked glycoprotein receptor that accounts for multiple heat-stable enterotoxin-binding proteins in the intestine

Guanylyl cyclase C (GC-C) is a newly discovered receptor found in the intestine, and possibly in other epithelia, that binds bacterial heat-stable enterotoxins (STa). The receptor has now been stably expressed in human embryonic 293 cells, which do not normally contain the receptor. Cyclic GMP concentrations are elevated 40-fold in response to 1 microM STa, and membranes obtained from the overproducing cells contain GC-C activity that can be stimulated about 9-fold by STa alone and an additional 1.4- to 2-fold by a combination of ATP and STa. The ATP effect does not appear to be due to enzyme activation, but instead to protection of GC-C against inactivation. Antibody raised against the carboxyl-terminal sequence of GC-C identified two major proteins (M(r) 140,000 and 160,000) in 293 cells expressing GC-C. Both proteins bound to wheat germ lectin-Sepharose, and N-glycosidase F treatment converted both forms to a single M(r) 120,000 protein, the size predicted from amino acid composition. The addition of high concentrations of tunicamycin to 293-GC-C cells also reduced the M(r) to 120,000, indicating that GC-C is an N-linked glycoprotein. When rat intestinal membranes or 293-GC-C cells were cross-linked with 125I-labeled STa, the major 125I-labeled protein complexes had M(r) ranging between 45,000 and 80,000. On immunoblots of rat intestinal membranes treated with a reducing agent, 3 major proteins of M(r) 65,000, 85,000, and 140,000 were specifically recognized by a GC-C antibody. However, under nonreducing conditions one major GC-C related protein appeared at a higher position (M(r) 230,000). Its mobility was reduced in the presence of STa, similar to rCG-C expressed in 293 cells. These data indicate that at least part of the lower M(r) STa-binding proteins found in intestinal extracts represent proteolytic products of GC-C.