Quantifying the Initial Unfolding of Bacteriorhodopsin Reveals Retinal Stabilization

The forces that stabilize membrane proteins remain elusive to precise quantification. Particularly important, but poorly resolved, are the forces present during the initial unfolding of a membrane protein, where the most native set of interactions is present. A high‐precision, atomic force microscopy assay was developed to study the initial unfolding of bacteriorhodopsin. A rapid near‐equilibrium folding between the first three unfolding states was discovered, the two transitions corresponded to the unfolding of five and three amino acids, respectively, when using a cantilever optimized for 2 μs resolution. The third of these states was retinal‐stabilized and previously undetected, despite being the most mechanically stable state in the whole unfolding pathway, supporting 150 pN for more than 1 min. This ability to measure the dynamics of the initial unfolding of bacteriorhodopsin provides a platform for quantifying the energetics of membrane proteins under native‐like conditions.     

N‐Terminally Truncated Amyloid‐β(11–40/42) Cofibrillizes with its Full‐Length Counterpart: Implications for Alzheimer's Disease

Amyloid‐β peptide (Aβ) isoforms of different lengths and aggregation propensities coexist in vivo. These different isoforms are able to nucleate or frustrate the assembly of each other. N‐terminally truncated Aβ_(11–40) and Aβ_(11–42) make up one fifth of plaque load yet nothing is known about their interaction with full‐length Aβ_(1–40/42). We show that in contrast to C‐terminally truncated isoforms, which do not co‐fibrillize, deletions of ten residues from the N terminus of Aβ have little impact on its ability to co‐fibrillize with the full‐length counterpart. As a consequence, N‐terminally truncated Aβ will accelerate fiber formation and co‐assemble into short rod‐shaped fibers with its full‐length Aβ counterpart. This has implications for the assembly kinetics, morphology, and toxicity of all Aβ isoforms.