Differential processing of guanylyl cyclase C along villus-crypt axis of rat small intestine

Many strains of enterotoxigenic Escherichia coli produce a heat-stable peptide enterotoxin (STa) that binds to the intestinal receptor guanylyl cyclase C (GC-C). STa receptors are structurally heterogeneous, but the molecular events causing this heterogeneity remain obscure. We examined the influence of cell position along the villus-crypt axis on STa receptor heterogeneity by fractionating EDTA-dissociated cells that detached in a villus-to-crypt direction. STa affinity labeling experiments revealed that the initially released villus "tip" fraction had four major STa binding proteins (STBPs), with relative molecular weight (M(r)) of 150,000, 135,000, 125,000, and 95,000, that did not react with a GC-C carboxy-terminal antibody. Yet succeeding villus cell fractions had major immunoreactive STBPs with M(r) of 275,000 and 250,000. Limited proteolysis of these larger GC-C isoforms produced 1) smaller STBPs that had M(r) similar to those in the initial villus fraction, 2) a 65,000 M(r) protein GC-C isoform that did not bind STa, and 3) elevated basal and STa-induced cyclase activity. Our data show that STBP structural heterogeneity in the intact intestine arises largely from multisite proteolytic processing of GC-C.