QBP1 interferes with Orb2 amyloid formation in vitro.

(A) Representative calorimetric traces for the interaction of full-length Orb2A with the QBP1 and SCR peptides. Traces in orange and pink correspond to the heat released upon injection of Orb2A into the ITC cell loaded with an excess of QBP1 or SCR, respectively, while black, blue, and red traces correspond to the Orb2A, QBP1, and SCR dilutions, respectively. (B) QBP1 but not SCR drastically reduced the turbidity at 405 nm of aged Orb2A PLD. (C) QBP1 reduces the quantity of amyloid formed as shown by measuring the CR bound to Orb2A PLD. The data are represented as the mean ± SEM: *p < 0.05 and ***p < 0.001 (One-way ANOVA and Tukey post-test). (D) Far-UV CD spectroscopy in the absence or presence of QBP1 or SCR indicates that QBP1 blocks protein precipitation over time of Orb2A PLD, but SCR does not. (E) Representative electron micrographs show that oligomers (asterisks) and amyloid fibers (arrows) of Orb2A PLD were drastically reduced when incubated with QBP1 (left) but not with the SCR (right). Scale bars: 1 μm. (F) The conformational polymorphism of Orb2A PLD is strongly diminished in the presence of a 1:10 molar excess of QBP1, which decreases the M frequency relative to the NM conformers (QBP1, n = 112 and SCR, n = 101). The underlying data for panels in this figure can be found in S1 Data.