ONE PEPTIDE, 2 TOPOLOGIES - STRUCTURE AND INTERCONVERSION DYNAMICS OFHUMAN UROGUANYLIN ISOMERS

The peptide hormone uroguanylin stimulates chloride secretion via acti vation of intestinal guanylyl cyclase C (GC-C). It is characterized by two disulfide bonds in a 1-3/2-4 pattern that causes the existence of two topological stereoisomers of which only one induces intracellular cGMP elevation. To obtain an unambiguous structure-function relations hip of the isomers, we determined the solution structure of the separa ted uroguanylin isoforms using NMR spectroscopy. Both isomers adopt we ll-defined structures that correspond to those of the isomers of the r elated peptide guanylin. Furthermore, the structure of the GC-C-activa ting uroguanylin isomer A closely resembles the structure of the agoni stic Escherichia coli heat-stable enterotoxin. Compared with guanylin isomers, the conformational interconversion of uroguanylin isomers is retarded significantly. As judged from chromatography and NMR spectros copy, both uroguanylin isoforms are stable at low temperatures, but ar e subject to a slow pH-dependent mutual isomerization at 37 degrees C with an equilibrium isomer ratio of approximately 1:1. The conformatio nal exchange is most likely under the sterical control of the carboxy- terminal leucine. These results imply that GC-C is activated by ligand s exhibiting the molecular framework corresponding to the structure of uroguanylin isomer A.