BACKGROUND Mutations in cardiac sodium channel Na(v)1.5 cause Brugada syndrome (BrS). MOG1 is a chaperone that binds to Na(v)1.5, facilitates Na(v)1.5 trafficking to the cell surface, and enhances the amplitude of sodium current I-Na. OBJECTIVE The purpose of this study was to identify structural elements involved in MOG1-Na(v)1.5 interaction and their relevance to the pathogenesis of BrS. METHODS Systematic analyses of large deletions, microdeletions, and point mutations, and glutathione S-transferases pull-down, co-immunoprecipitation, cell surface protein quantification, and patch-clamping of I-Na were performed. RESULTS Large deletion analysis defined the MOG1-Na(v)1.5 interaction domain to amino acids S-476-H-585 of Na(v)1.5 Loop I connecting transmembrane domains I and II. Microdeletion and point mutation analyses further defined the domain to F530T531F532R533R534R535. Mutations F530A, F532A, R533A, and R534A, but not T531A and R535A, significantly reduced MOG1-Na(v)1.5 interaction and eliminated MOG1-enhanced I-Na. Mutagenesis analysis identified D24, E36, D44, E53, and E101A of MOG1 as critical residues for interaction with Na(v)1.5 Loop I. We then characterized 3 mutations at the MOG1-Na(v)1.5 interaction domain: p.F530V, p.F532C, and p.R535Q reported from patients with long QT syndrome and BrS. We found that p.F532C reduced MOG1-Na(v)1.5 interaction and eliminated MOG1 function on I-Na; p.R535Q is also a loss-of-function mutation that reduces I-Na amplitude in a MOG1-independent manner, whereas p.F530V is benign as it does not have an apparent effect on MOG1 and I-Na. CONCLUSION Our findings define the MOG1-Na(v)1.5 interaction domain to a 5-amino-acid motif of F530T531F532R533R534 in Loop I. Mutation p.F532C associated with BrS abolishes Na(v)1.5 interaction with MOG1 and reduces MOG1-enhanced I-Na density, thereby uncovering a novel molecular mechanism for the pathogenesis of BrS.