Guanylyl cyclase C (GC-C) is a member of particulate guanylyl cyclases, discovered primarily as the target of a family of heat stable enterotoxins (ST), produced by enterotoxigenic Escherichia coli (ETEC). ST is acknowledged as a prime cause of traveller’s diarrhea and the leading cause of child mortality under the age of 5 years in developing nations. The bacterial expression of ST peptides represents molecular mimicry where the pathogen has exploited a gastrointestinal tract-signaling pathway to disperse and propagate. GC-C is primarily expressed on the apical or the brush border membranes of intestinal epithelial cells. GC-C agonists elaborated in the gastrointestinal tract are a family of guanylin peptides, which are responsible for maintaining fluid-ion homeostasis, essential for normal gut physiology. The signal of liigand binding to the extracellular domain of GC-C is transduced to the catalytic guanylyl cyclase domain, which results in production of intracellular cGMP. The elevated levels of cGMP influence multiple downstream targets, which finally regulate ion-flux through the transporters present on the membrane of an enterocyte. The ST peptide, a GC-C superagonist, produces physiologically abnormal levels of cGMP that manifest as secretory diarrhea. The purview of GC-C misregulation was confined to the notion of its hyperactivation caused by ETEC infection and the ensuing diarrhea. Recently, two seminal studies widened the scope of pathologies associated with GC-C. Studies described point mutations in GUCY2C, which were associated with human disease. One study identified a Norwegian family whose members demonstrated a dominantly inherited syndrome of frequent diarrhea associated with hyperactive GC-C. Following this study, inactivating mutations in GC-C in a small Bedouin population was reported. The current study reports the molecular phenotypes associated with the first germ line mutations in GC-C that result in a severe form of congenital sodium diarrhea. Our collaborators from Austria (Thomas Muller & Andreas Janecke, Department of Pediatrics Innsbruck Medical University) communicated to us their study of patients who had clinical diagnosis of congenital sodium diarrhea, with proportionally high fecal sodium loss, metabolic acidosis and dehydration. Exome sequencing in a cohort of 6 unrelated patients revealed four heterozygous missense mutations in GC-C (R792S, L775P, K507E, N850D). Novel GC-C mutations were de novo spontaneous mutations with the carrier being the only affected family member in contrast to the previous two reports with familial history. Biochemical characterization revealed that the mutants (GC-CR792S, GC-CL775P) were constitutively active with GC-CR792S, GC-CK507E, and GC-CN850D showing further stimulation upon treatment with ST and guanylin family of peptides. Interestingly, there was no change in the binding affinities of the ligands for the mutant receptors compared to wild type. However, a significant decrease (ranging from 10-100 fold) in ligand EC50 for the mutant GC-C receptors was prominent. The in vitro assays suggested that the mutations occupying different domains of GC-C might have resulted in distinct structural consequences reflected in the repertoire of phenotypes that were observed. The results presented in this thesis illustrate the molecular basis of the severe form of congenital diarrhea associated with the GC-C gain-of-function mutations. This study has also elaborated our understanding of the regulation of GC-C activity by its various domains.