Abstract
Mixtures of “template” and “adder” proteins self-assemble into large amyloid fibers of varying morphology and modulus. Fibers range from low modulus, rectangular cross-sectioned tapes to high modulus, circular cross-sectioned cylinders. Varying the proteins in the mixture can elicit “in-between” morphologies, such as elliptical cross-sectioned fibers and twisted tapes, both of which have moduli in-between rectangular tapes and cylindrical fibers. Experiments on mixtures of proteins of known amino acid sequence show that control of the large amyloid fiber morphology is dependent on the amount of glutamine repeats or “Q-blocks” relative to hydrophobic side chained amino acids such as alanine, isoleucine, leucine, and valine in the adder protein. Adder proteins with only hydrophobic groups form low modulus rectangular cross-sections and increasing the Q-block content allows excess hydrogen bonding on amide groups that results in twist and higher modulus. The experimental results show that large amyloid fibers of specific shape and modulus can be designed and controlled at the molecular level.
Highlights
Amyloids are self-assembled protein materials containing β-sheets
Functional amyloids occur as rigid protofibrils about 2–4 nm high, 10–30 nm wide, and >100 nm long with β-sheets oriented perpendicular to the protofibril axis to yield what is traditionally called the “cross-β” protein secondary structure [1,3,4,5]
Large fibrils will bundle into large amyloid fibers in different ways depending on the constituent proteins in the mixture or the self-assembling solution conditions [21,23,26]
Summary
Amyloids are self-assembled protein materials containing β-sheets. While the most common context for amyloids is in neurodegenerative diseases, there is another class of amyloid that performs beneficial functions in nature called “functional” amyloids [1,2]. Functional amyloids occur as rigid protofibrils about 2–4 nm high, 10–30 nm wide, and >100 nm long with β-sheets oriented perpendicular to the protofibril axis to yield what is traditionally called the “cross-β” protein secondary structure [1,3,4,5]. Examples include egg silks of the Chrysopidae family, barnacle cement, and bacterial and fungal hyphae [2,6,7,8,9,10]. These natural functional amyloids have been shown to display toughness as evidenced by a “sawtooth” pattern in atomic force microscopy (AFM) force-distance curves [11]. Functional amyloids have become an inspiration for the burgeoning field of using amyloid protofibrils for high performance materials [1,12,13,14,15]
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