title: α/β coiled coils.
abstract: Coiled coils are the best-understood protein fold, as their backbone structure can uniquely be described by parametric equations. This level of understanding has allowed their manipulation in unprecedented detail. They do not seem a likely source of surprises, yet we describe here the unexpected formation of a new type of fiber by the simple insertion of two or six residues into the underlying heptad repeat of a parallel, trimeric coiled coil. These insertions strain the supercoil to the breaking point, causing the local formation of short β-strands, which move the path of the chain by 120° around the trimer axis. The result is an α/β coiled coil, which retains only one backbone hydrogen bond per repeat unit from the parent coiled coil. Our results show that a substantially novel backbone structure is possible within the allowed regions of the Ramachandran space with only minor mutations to a known fold.
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Marcus D
authors.identifier: http://orcid.org/0000-0001-6937-5677
authors.initials: MD
authors.lastname: Hartmann
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Claudia T
authors.initials: CT
authors.lastname: Mendler
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Bassler
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Ioanna
authors.initials: I
authors.lastname: Karamichali
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Oswin
authors.initials: O
authors.lastname: Ridderbusch
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Andrei N
authors.identifier: http://orcid.org/0000-0002-1959-4836
authors.initials: AN
authors.lastname: Lupas
authors.affiliation: Department of Protein Evolution, Max Planck Institute for Developmental Biology, Tübingen, Germany.
authors.forename: Birte
authors.initials: B
authors.lastname: Hernandez Alvarez
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D018360:Crystallography, X-Ray; D006094:Gram-Positive Bacteria / Q000737:chemistry / Q000235:genetics; D006860:Hydrogen Bonding; D008958:Models, Molecular; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D001426:Bacterial Proteins
keywords: biophysics; fiber; heptad repeat; none; sequence periodicity; structural biology; supercoil; α-helix