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The self-assembly of biological molecules into ordered nanostructures is an attractive method for fabricating novel nanomaterials. Nucleic acid-based nanostructures suffer from limitations to functionalization and stability. Alternatively, protein-based nanostructures have advantageous chemical properties, but design facility lags behind that of nucleic acids. Structurally defined fibrils engineered from β-solenoid proteins (BSPs) form under mild conditions [Peralta, M. D. R., et al. (2015) ACS Nano 9, 449-463] and are good candidates for novel nanomaterials because of the defined sequence-to-structure relationship and tunable properties. Here, the stability of two types of engineered fibrils was examined using circular dichroism spectroscopy, transmission electron microscopy, and electrophoresis. Both are stable to at least 90 °C, and one survives autoclaving. They are stable toward organic solvents, urea, and pH extremes. One is even stable in 2% sodium dodecyl sulfate with heating. The fibrils show variable resistance to proteolytic digestion: one is resistant to trypsin, but chymotrypsin and proteinase K degrade both. These results show that BSPs have excellent potential for bottom-up design of rugged, functional, amyloid-based nanomaterials.

We present estimates of ultimate tensile strength (UTS) for two engineered β -solenoid protein mutant fibril structures (spruce budworm and Rhagium inquisitor antifreeze proteins) derived from sonication-based measurements and from force pulling molecular dynamics simulations, both in water. Sonication experiments generate limiting scissioned fibrils with a well-defined length-to-width correlation for the mutant spruce budworm protein and the resultant UTS estimate is 0.66 ± 0.08 GPa. For fibrils formed from engineered R. inquisitor antifreeze protein, depending upon geometry, we estimate UTSs of 3.5 ± 3.2–5.5 ± 5.1 GPa for proteins with interfacial disulfide bonds, and 1.6 ± 1.5–2.5 ± 2.3 GPa for the reduced form. The large error bars for the R. inquisitor structures are intrinsic to the broad distribution of limiting scission lengths. Simulations provide pulling velocity-dependent UTSs increasing from 0.2 to 1 GPa in the available speed range, and 1.5 GPa extrapolated to the speeds expected in the sonication experiments. Simulations yield low-velocity values for the Young's modulus of 6.0 GPa. Without protein optimization, these mechanical parameters are similar to those of spider silk and Kevlar, but in contrast to spider silk, these proteins have a precisely known sequence-structure relationship.