Fig. 1: Normal HTT RNA with 7 CAG repeats unfolds cooperatively.
From: Stick-slip unfolding favors self-association of expanded HTT mRNA
A RNAs corresponding to the CAG repeat region of human HTT exon 1 were attached to optically trapped beads via dsDNA handles. fHTTn mRNAs contain n = 7, 12, 20, or 40 CAG triplets (red line) plus flanking CCGCAA(CCG)7 repeats (gold line). B Example stretch (black, S1) and relax (red, R1) FEC for fHTT7 at 90 nm/s show unfolding/refolding transitions near 11.5 pN. Inset: slip transitions at low tension. Dashed curves represent eWLC models for folded (black), unfolded (gray) and slipped (blue) states. The expected ∆Lc for transitioning from the fully folded to the fully unfolded state is 26.0 nm. See Supplementary Fig. 4 for details. S1 and R1 are plotted with an offset for clarity. C Contour lengths of slipped intermediates before the main unfolding transition from eWLC fits (blue dashed line in (B)) relative to the folded state (eWLCMFE—black dashed line in (B)). Red line, sum of 5 Gaussians; black dashed line, individual Gaussians. The histogram was fit using GraphPad Prism version 10.2.3 for Mac OS X. Labeled peak averages correlate with structures predicted by mfold54 and footprinting10: (i) no unpaired CAG triplet (–1.76 ± 0.04 nm), (ii) reference MFE structure with one single-stranded CAG triplet (0.19 ± 0.06 nm), (iii) three unpaired CAG triplets (3.87 ± 0.02 nm). RNA schematics were prepared with RNAcanvas55. D Distributions of forces for unfolding (black) and refolding (red) transitions, from experiments as in (B). (N = 286 FECs from 48 molecules). The transitions represent slippage between intermediate structures (5–10 pN) and complete unfolding above 10 pN.
