Proteomic and biochemical analyses unveil tight interaction of ataxin-3 with tubulin

Ataxin-3 consists of an N-terminal globular Josephin domain and an unstructured C-terminal region containing a stretch of consecutive glutamines that triggers an inherited neurodegenerative disorder, spinocerebellar ataxia type 3, when its length exceeds a critical threshold. The pathology results from protein misfolding and intracellular accumulation of fibrillar amyloid-like aggregates. Plenty of work has been carried out to elucidate the protein's physiological role(s), which has shown that ataxin-3 is multifunctional; it acts as a transcriptional repressor, and also has polyubiquitin-binding/ubiquitin-hydrolase activity. In addition, a recent report shows that it participates in sorting misfolded protein to aggresomes, close to the microtubule-organizing center. Since a thorough understanding of the protein's physiological role(s) requires the identification of all the molecular partners interacting with ataxin-3, we pursued this goal by taking advantage of two-dimensional chromatography coupled to tandem mass spectrometry. We found that different ataxin-3 constructs, including the sole Josephin domain, bound alpha- and beta-tubulin from soluble rat brain extracts. Coimmunoprecipitation experiments confirmed this interaction. Also, normal ataxin-3 overexpressed in COS7 cultured cells partially colocalized with microtubules, whereas an expanded variant only occasionally did so, probably due to aggregation. Furthermore, by surface plasmon resonance we determined a dissociation constant of 50-70nM between ataxin-3 and tubulin dimer, which strongly supports the hypothesis of a direct interaction of this protein with microtubules in vivo. These findings suggest an involvement of ataxin-3 in directing aggregated protein to aggresomes, and shed light on the mode of interaction among the different molecular partners participating in the process.