Figure 15. Model of structural changes observed in HTT proteins upon temperature change.

The N17 (orange) and polyproline (blue) regions are depicted as alpha- and PPII helices, respectively, while the polyglutamine region is illustrated as a domain which can adopt temperature- and polyQ-dependent conformations (red). The epitopes of the 2B7 and MW1 antibodies are graphically illustrated. A. The N-terminal region of a HTT protein bearing a wild type (e.g. Q16) polyQ expansion can adopt variable degrees of structure in the polyQ region, with alpha-helical content being higher at lower temperatures. At higher temperatures, alpha-helical content is lower and results in a 1: ≤1 ratio of 2B7Tb-MW1D2 bound to HTT. At lower temperatures, this ratio becomes 1: ≥1 due to increased alpha-helical content in the polyglutamine region, an increased number of epitopes for the acceptor-labeled MW1 antibody and, consequently, an increase in TR-FRET signal. B. In the N-terminal region of a HTT protein bearing a mutant (e.g. Q55) polyQ expansion, the polyQ region adopts a higher alpha-helical content relatively to wild type HTT, a relatively larger number of epitopes for the acceptor-labeled MW1 antibody are present and, consequently, TR-FRET signals are higher. The significant alpha-helical structure of the mutant polyQ region is less influenced by temperature, the number of MW1 epitopes remains relatively constant and therefore FRET between donor and acceptor labels is relatively unchanged.