Abstract
The polyglutamine expansion in huntingtin protein causes Huntington's disease. Here, we investigated structural and biochemical properties of huntingtin and the effect of the polyglutamine expansion using various biophysical experiments including circular dichroism, single-particle electron microscopy and cross-linking mass spectrometry. Huntingtin is likely composed of five distinct domains and adopts a spherical α-helical solenoid where the amino-terminal and carboxyl-terminal regions fold to contain a circumscribed central cavity. Interestingly, we showed that the polyglutamine expansion increases α-helical properties of huntingtin and affects the intramolecular interactions among the domains. Our work delineates the structural characteristics of full-length huntingtin, which are affected by the polyglutamine expansion, and provides an elegant solution to the apparent conundrum of how the extreme amino-terminal polyglutamine tract confers a novel property on huntingtin, causing the disease.
Highlights
Huntingtin is the entire large protein product (>350 kDa MW) of the Huntingtin gene (HTT previously Huntington’s disease (HD))
The circular dichroism (CD) spectra (Figure 1B) of all of the huntingtins are consistent with a predominant a-helical secondary structure (Liu and Rost, 2003; Rost et al, 1994) (Figure 1A), with typical minima at 222 and 208 nm and a positive peak at 195 nm, and all exhibited the same irreversible thermal denaturation pattern, with secondary structure stable up to ~38–40 ̊C, a gradual slow denaturation as the temperature is increased to 65–70 ̊C, followed by aggregation and some precipitation (Figure 1C)
We investigated the proposal that huntingtin’s shape may enable a structural impact of the amino-terminal polyglutamine tract, by performing single-particle electron microscopy (EM) of recombinant human huntingtins with polyglutamine tract lengths of 23- and 78-residues
Summary
Huntingtin is the entire large protein product (>350 kDa MW) of the Huntingtin gene (HTT previously HD). If expanded above 38-residues, this mutation causes Huntington’s disease (HD), a dominant neurodegenerative disorder (Huntington’s Disease Collaborative Research Group, 1993). The strong correlation between the size of the expanded repeat and the age at diagnosis of HD motor, cognitive and psychiatric symptoms shows that CAG repeat-size is the primary determinant of the rate of the disease progression (Brinkman et al, 1997; Snell et al, 1993). This biological relationship provides a human patient-based rationale for delineating the HD disease-trigger in studies with an allelic series
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