Glutamine Side-Chain to Main Chain Hydrogen Bonds Can be used to Design Single Alpha-Helices that are Stable at Room Temperature

The genetic instability of polyCAG repeats is the cause of polyglutamine (polyQ) tract expansions, which is a potential molecular basis of a group of disorders known as polyQ diseases. PolyQ tracts are common sequences rich in glutamine residues mainly found in intrinsically disordered regions of proteins related to transcription. Their expansion has been hypothesized to induce a conformational change affecting their solubility that results in the formation of fibrillar aggregates in specific tissues of patients affected by polyQ diseases. With most in vitro studies reporting a collapsed random coil arrangement with no major conformational shifts upon expansion, recent studies have shown a certain propensity to form helices that increases with tract length. In physiological conditions, this effect may be enhanced by the interactions with binding partners as well as by partitioning proteins bearing polyQ tracts in phase-separated protein condensates. Such an environment can favour the formation of the side-chain (i) to main-chain (i-4) hydrogen bonds that we recently reported as a stabilizing factor of polyQ helices (Escobedo, A. et al. Nat. Commun. 10, 2034 (2019)). We here show that this is a common feature of polyQ tracts, unveil bulkiness and hydrophobicity of the accepting residue type as the main factors affecting the strength of the interaction, and exploit this knowledge to generate uncharged single a-helices that are stable at room temperature and therefore suitable for protein and biomaterials design.