Huntington’s disease is caused by mutations that expand a polyglutamine (polyQ) repeat sequence within huntingtin from less than (typically) 25 glutamines to greater than 36. The longer glutamine lengths cause huntingtin to misfold and aggregate as a key feature of disease pathology.  While aggregation correlates with disease onset, recent studies have identified a closer correlation between an abnormal monomer conformation and dysfunction. However, knowledge of how polyglutamine lengths modulate the monomer conformation and confer detrimental impacts to the cell is at best fragmentary. Here we describe our project to define which proteins in the proteome preferentially interact with the polyQ-expanded monomer, and our strategy to investigate the structural properties of the monomer. We applied immunoprecipitation to pull down binding partners of monomeric GFP-tagged huntingtin and compared the differences between a non-pathogenic (25Q) and pathogenic (46Q) variant versus a GFP only control. Mass spectrometry identified 811 protein-binding partners specific to huntingtin, with five proteins preferentially bound to the polyQ-expanded monomer. The RNA binding protein Fus was the most significant interactor with pathogenic huntingtin, which has previously been linked to motor neuron disease, suggesting a possible synergy in mechanisms of toxicity in these two neurodegenerative diseases that will be further explored.  We describe also our strategy to probe the conformation of mutant (36-46Q) vs non-mutant (16-25Q) huntingtin monomers by NMR using two approaches.  First we will insert lanthanide probes at different sequence positions of huntingtin to selectively disperse otherwise overlapping polyQ resonances in a distance and orientation-dependent manner and use the information to build a map of the three dimensional structure. Second we will examine polyQ resonances by NMR on huntingtin directly expressed or “transfected” into mammalian cells, which enables the observation of huntingtin structure in context of the cellular mechanisms that engage with the Htt and possibly modify its conformation.