Huntington’s disease (HD) is caused by mutant huntingtin containing a polyglutamine expansion from a normal range or typically less than 25 glutamines to greater than 36. While this expansion is known to cause aggregation, which is a key marker of pathology, recent studies suggest that toxicity to cells arises prior to the aggregation process pointing to the monomer as the key initiator of pathogenesis.  Here we describe our work in progress to characterize the interactome of the mutant monomer in cells relative to the wild-type monomer as potential targets for how the monomer acquires a gain-of-toxic function; and how the pathogenic polyQ length influences the conformation of the monomer structure to identify possible mechanisms for this gain of toxic function.  GFP-Trap immunoprecipitation on soluble cell extracts and label-free mass spectrometry methods were used to identify 32 proteins that preferentially bound to pathogenic huntingtin (46Q-GFP) compared to the non-pathogenic (25Q-GFP). These included RNA-binding protein Fus, antioxidant Prdx6, RNA ligase Gars, neuronal apoptotic regulator Pebp1 and histone subunit Hist1H4a. The functional implication of these genes will be tested using RNAi knockdown. To investigate the structure, we have applied NMR and mass spectrometry using two approaches.  First, because huntingtin is thought to adopt an intrinsically disordered structure, we have used lanthanide probes embedded within the sequence of huntingtin to resolve the polyQ resonances by HSQC and define changes in localized dynamics, such as collapsed versus expanded disordered structures.  Second, we are in the process of applying hydrogen-deuterium exchange to identify regions of collapse-disordered structure predicted to arise from non-specific side-chain-amide backbone hydrogen bonding patterns.