The hunt for huntingtin function: interaction partners tell many different stories

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormally elongated polyglutamine (polyQ) tract in the large protein huntingtin (htt). Currently, both the normal function of htt in neurons and the molecular mechanism by which the expanded polyQ sequence in htt causes selective neurodegeneration remain elusive. Research in past years has identified several htt-interacting proteins such as htt-interacting protein 1, Src homology region 3-containing Grb2-like protein 3, protein kinase C and casein kinase substrate in neurons 1, htt-associated protein 1, postsynaptic density-95, FIP-2 (for 14.7K-interacting protein), specificity protein 1 and nuclear receptor co-repressor. These proteins play roles in clathrin-mediated endocytosis, apoptosis, vesicle transport, cell signalling, morphogenesis and transcriptional regulation, suggesting that htt is also involved in these processes.