Huntingtin affinity for partners is not changed by polyglutamine length: aggregation itself triggers aberrant interactions.

Huntington's disease (HD) is caused by the expansion mutation above a length threshold of a polyglutamine (polyQ) stretch in the huntingtin (Htt) protein. Mutant Htt (mHtt) pathogenicity is proposed to rely on its malfunction and propensity to misfold and aggregate. Htt has scaffolding properties and has been reported to interact with hundreds of partners. Many interactors show apparent increased or decreased affinity (dysinteraction) for mHtt, which may account for selective malfunctions and striatal degeneration in HD. These dysinteractions are proposed to result from mutant polyQ conformational changes that remain elusive. To date, dysinteractions have only been studied using semi-quantitative techniques with their outcome potentially influenced by the presence of mHtt aggregates. Therefore, the molecular mechanism underlying these dysinteractions remains to be determined. Here, we have used purified proteins devoid of aggregates to quantify the interaction of normal and mHtt with two partners: SH3GL3, reported to have increased binding to mHtt, and the 2B4 antibody, a model partner. Using surface plasmon resonance and pull-down techniques, we show that in the absence of aggregation polyQ length has no effect on Htt interactions. We demonstrate that the presence of aggregates affects the spatial distribution and solubility of Htt partners and strongly influences the outcome of pull-down experiments. Our results show that expanded polyQ per se does not alter Htt interactions and suggest that aggregated mHtt form molecular platforms that influence the Htt interacting network. Modulating mHtt aggregation could thus have beneficial effects on specific cellular pathways deregulated in HD.