Huntington's disease (HD), a neurodegenerative disorder characterized by loss of striatal neurons, is caused by an expansion of a polyglutamine tract in the HD protein huntingtin. HAP1 was initially identified through a two-hybrid library screening; the binding of HAP1 to huntingtin correlated with the expansion of the polyglutamine tract. HAP1 also interacts with two cytoskeletal proteins (dynactin and pericentriolar autoantigen protein 1), suggesting that HAP1 may play a role in vesicular trafficking or organelle transport. HAP1 is also involved with the huntingtin-enhanced BDNF transport along the cellular microtubles. Attenuation of this process led to the loss of neurotrophic support and neuronal toxicity, which suggests that loss of this function might contribute to pathogenesis. Several alternatively spliced isoforms have been described for HAP1.
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