Tissue transglutaminase selectively modifies proteins associated with truncated mutant huntingtin in intact cells

The cause of Huntington's disease (HD) is a pathological expansion of the p olyglutamine domain within the N-terminal region of huntingtin. Neuronal in tranuclear inclusions and cytoplasmic aggregates composed of the mutant hun tingtin within certain neuronal populations are a characteristic hallmark o f HD. However, how the expanded polyglutamine repeats of mutant huntingtin cause HD is not known. Because in vitro expanded polyglutamine repeats are excellent glutaminyl-donor substrates of tissue transglutaminase (tTG), it has been hypothesized that tTG may contribute to the formation of these agg regates in HD. However, an association between huntingtin and tTG or modifi cation of huntingtin by tTG has not been demonstrated in cells. To examine the interactions between tTG and huntingtin human neuroblastoma SH-SY5Y cel ls were stably transfected with full-length huntingtin containing 23 (FL-Q2 3) (wild type) or 82 (FL-Q82) (mutant) glutamine repeats or a truncated N-t erminal huntingtin construct containing 23 (Q23) (wild type) or 62 (Q62) (m utant) glutamine repeats. Aggregates were rarely observed in the cells expr essing full-length mutant huntingtin, and no specific colocalization of ful l-length huntingtin and tTG was observed. In contrast, in cells expressing truncated mutant huntingtin (Q62) there were numerous complexes of truncate d mutant huntingtin and many of these complexes co-localized with tTG. Howe ver, the complexes were not insoluble structures. Further, truncated huntin gtin coimmunoprecipitated with tTG, and this association increased when tTG was activated. Activation of tTG did not result in the modification of eit her truncated or full-length huntingtin, however proteins that were associa ted with truncated mutant huntingtin were selectively modified by tTG. This study is the first to demonstrate that tTG specifically interacts with a t runcated form of huntingtin, and that activated tTG selectively modifies mu tant huntingtin-associated proteins. These data suggest that proteolysis of full-length mutant huntingtin likely precedes its interaction with tTG and this process may facilitate the modification of huntingtin-associated prot eins and thus contribute to the etiology of HD. (C) 2001 Academic Press.