TSR1 and inflammatory bowel disease: Mutation of Leu456 to valine at the TSR6-TSR1 interface had a modest effect in this assay despite its coupling to inflammatory bowel disease (46) whereas there was a severe defect in FP secretion for the triple mutant L47A/L58A/L456V affecting both the TB-TSR4 and the TSR6-TSR1 interaction (Figure 5C).