The NaV1.5 voltage-gated Na+ channel encoded by SCN5A is the fundamental component of macromolecular protein complexes that initiate the cardiac action potential. Abnormal NaV1.5 function is a prominent substrate for inherited and acquired forms of cardiac arrhythmias, reflected by a staggering array of identified NaV1.5 mutations. A small subset of these are associated with dilated cardiomyopathy but the underlying mechanisms are not known. A leading hypothesis, that the arrhythmias drive the cardiomyopathy, cannot explain why most arrhythmogenic NaV1.5 mutations do not cause cardiomyopathy nor why knockout of the NaV1.5 interacting protein FGF13 leads to arrhythmias yet is protective for pressure overload-induced heart failure (HF) despite associated NaV1.5 dysfunction. Moreover, HF from other causes leads to pathological remodeling that disrupts regulation of the VGSC macromolecular complex and increases arrhythmia risk through mechanisms that are poorly understood. Complicating mechanistic insight is that there are different NaV1.5 pools defined by distinct subcellular localizations with the cardiomyocyte, each hypothesized to have protein partners that uniquely define the distinct pools and confer specific channel properties and functions. However, the critical partners remain poorly understood because of challenges of low throughput “favorite” candidate approaches. We propose an unbiased multilevel discovery strategy, employing newly developed second generation proximity labeling tools, novel mouse models, coupled with carefully calibrated cross comparisons designed to increase the specificity of our findings. Exploiting the expertise from two labs with individual and collaborative track records applying a large tool set to dissect complex physiologic mechanisms and define perturbations in pathological states, we propose adaptable candidate validation approaches to establish a comprehensive picture of NaV1.5 interactomes under physiological states and when perturbed by disease. With these innovative approaches we propose to: 1) Define the static and dynamic NaV1.5 channel interactomes and “neighborhoods” within distinct subcellular pools; 2) Elucidate how HF alters the NaV1.5 microenvironment; and 3) Determine the HF-protective effects for ablation of the NaV1.5 interactor, FGF13. With these aims, our goals are to define the contributions of the NaV1.5 macromolecular to development and progression of HF and its associated arrhythmias and to unravel how HF perturbs the NaV1.5 complex to increase arrhythmia risk and exacerbate HF in a vicious cycle.