Figure 2. Nanomechanical analysis of polyQ tracts and a non-amyloidogenic IDP (VAMP2).

(A) ΔL _c (left) and F (right) histograms for pFS-2 polyproteins carrying polyQ tracts. The sub-threshold Q₁₉ (front row) only shows NM conformers (orange bars; represented in the force histograms below the force sensitivity of our AFM: F∼20 pN). Familial-disease mutations of this protein (expanded polyQs: Q₃₅ and Q₆₂) exhibit conformational polymorphism that ranges from NM conformers to M conformers (red bars), the latter class including some hM conformers (F≥400 pN, i.e., likely toxic conformers according to our hypothesis, see text). It should be noted that the longer the polyQ tract, the greater the conformational polymorphism and the more hM conformers found. The inhibitor QBP1 (20 µM [42]) reduces this polymorphism and abolishes the hM conformers. The inset shows an example of a hM conformer of Q₆₂. Note that the hM class of conformers is a subset of the M set so that the percentage of hM conformers (respect to the total number of molecules sampled, n) is included into that of M (this also applies to the remainder main figures of this work as well as to Table 1). TEM images of the amyloid fibers formed by the corresponding carrier-guest proteins (not the whole pFS polyprotein) are shown on the right, highlighting the relationship between hM conformers and amyloidosis. From bottom to top, the scale bars correspond to 0.6, 0.3, 0.3, and 0.6 µm, respectively. (B) SMFS analysis of pFS-2+VAMP2. This non-amyloidogenic IDP [35] does not show conformational polymorphism. The scale bar from the TEM image on the right corresponds to 0.6 µm. Only data from the guest protein, and not from the carrier, are plotted in the histograms presented in this figure and Figures 3– 5.