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.
Highlights
Introduction aHelical coiled coils are ubiquitous protein domains, found in a wide range of structural and functional contexts (Lupas, 1996)
We have a long-standing interest in trimeric autotransporter adhesins (TAA), fibrous proteins of the Gram-negative bacterial surface (Bassler et al, 2015; Hartmann et al, 2012; Hoiczyk et al, 2000; Szczesny and Lupas, 2008), whose domains we routinely fuse to stabilizing adaptor coiled coils for biochemical and biophysical study (Deiss et al, 2014; Hernandez Alvarez et al, 2008)
As part of that study, we identified a putative TAA in Actinobacillus actinomycetemcomitans, OMP100, which carries insertions of 2 and Biophysics and structural biology of 3 residues within the heptad repeats of its stalk
Summary
Helical coiled coils are ubiquitous protein domains, found in a wide range of structural and functional contexts (Lupas, 1996). The structure of coiled coils is understood at a level unrivaled by any other fold They consist of at least two a-helices, wound into superhelical bundles and held together by a mostly hydrophobic core. A range of other periodicities is accessible to coiled coils, which is only restrained by the periodicity of the unperturbed a-helix (Gruber and Lupas, 2003) This restraint is responsible for the supercoiling of the bundle: As an ideal, straight a-helix has a periodicity of about 3.63 residues per turn, the heptad coiled coil has a left-handed twist to reduce the periodicity to 3.5 residues per turn with respect to the bundle axis.
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