Tandem Repeat Proteins (TRPs) possess the simplest sequence to structure relationship in full length proteins due to a lack of globular folding. Moreover, their folding is dictated by relatively few conserved amino acid positions, which interact in an inter-repeat fashion. Certain TRPs, from the leucine rich variant and HEAT repeat families, have been shown to fold in a helical fashion, with each successive repeat rotating and translating axially relative to the previous repeat. Owing to these characteristics, we posited that a single repeat module from either of these families could self-assemble into a superhelical nanotube. The chemically synthesized peptides LRV_M3Δ1 and HEAT_R1 were shown to self-assemble into high aspect-ratio helical filaments with ~9 nm diameters. Cryo-electron microscopy was employed, following lower resolution biophysical analysis, to analyze the structure of the filaments. Cryo-EM maps were used to generate three-dimensional reconstructions and atomic models at resolutions of 4.4 Å and 6.0 Å for LRV_M3Δ1 and HEAT_R1 respectively. Despite a close similarity in the repeat backbone, the number of subunits in each helical turn, the helical pitch, and the superhelical handedness were different for both peptides. These differences can be rationalized by a few key axial and lateral interactions and highlight the need for high resolution structural analysis. Importantly, the nanotubes have independently addressable and chemically distinguishable concave and convex surfaces, that could facilitate tailoring of these structures to applications in selective recognition, transport, and release.

Design of peptide nanotubes from tandem repeat protein modules

2019 Spring National Meeting

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