The rational design of linear peptides that assemble controllably and predictably in water is challenging. Short sequences must encode unique target structures and avoid alternative states. However, the non-covalent forces that stabilize and discriminate between states are weak. Nonetheless, for α-helical coiled-coil assemblies considerable progress has been made in rational de novo design. In these, sequence repeats of nominally hydrophobic (h) and polar (p) residues, hpphppp, direct the assembly of amphipathic helices into dimeric to tetrameric bundles. Expanding this pattern to hpphhph can produce larger α-helical barrels. Here, we show that pentameric to nonameric barrels are accessed by varying the residue at one of the h sites. In peptides with four L/I–K–E–I–A–x–Z repeats, decreasing the size of Z from threonine to serine to alanine to glycine gives progressively larger oligomers. X-ray crystal structures of the resulting α-helical barrels rationalize this: side chains at Z point directly into the helical interfaces, and smaller residues allow closer helix contacts and larger assemblies.
Bibliographical noteFunding Information:
WMD, GGR and DNW were supported by a European Research Council Advanced Grant (340764). FOJM is supported by the Bristol Chemical Synthesis Centre for Doctoral Training funded through the EPSRC (EP/G036764). KLS is supported by the South West Biosciences Doctoral Training Partnership through the BBSRC (BB/M009122/1). DNW is supported by a BBSRC responsive-mode grant (BB/R00661X/1). We thank the University of Bristol School of Chemistry Mass Spectrometry Facility for access to the EPSRC-funded Bruker Ultraex MALDI-TOF instrument (EP/K03927X/1) and BrisSynBio for access to the BBSRC-funded BMG Labtech Clariostar Plate Reader (BB/ L01386X/1). We would like to thank Diamond Light Source for access to beamlines I03, I04, I04-1 and I24 (Proposal 12342 & 23269), and for the support from the macromolecular crystallography staff.
© The Royal Society of Chemistry.