Figure 1: Structure and synthesis of human guanylin peptides. Guanylin and uroguanylin have 3 exons and 2 introns. Human preproguanylin has 115, proguanylin 94, and guanylin 15 amino acids. Human preprouroguanylin has 112, prouroguanylin 86, and uroguanylin 16 amino acids. Structures of human guanylin and uroguanylin are compared to the structure of heat-stable enterotoxin of E. coli (STa) at the bottom of the figure. Open circles show identical amino acids in both peptides. Solid circles represent amino acids responsible for peptide sensitivity to chymotrypsin.