DIFFERENTIAL PROCESSING OF GUANYLYL CYCLASE-C ALONG VILLUS-CRYPT AXISOF RAT SMALL-INTESTINE

Many strains of enterotoxigenic Escherichia coli produce a heat-stable peptide enterotoxigenic (STa) that binds to the intestinal receptor g uanylyl 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 e xperiments revealed that the initially released villus ''tip'' fractio n 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 r eact 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) sma ller STBPs that had M-r similar to those in the initial villus fractio n, 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 ST EP structural heterogeneity in the intact intestine arises largely fro m multisite proteolytic processing of GC-C.