title: Beta-strand flipping and slipping triggered by turn replacement reveal the opportunistic nature of beta-strand pairing.
abstract: We investigated how the register between adjacent beta-strands is specified using a series of mutants of the single-layer beta-sheet (SLB) in Borrelia OspA. The single-layer architecture of this system eliminates structural restraints imposed by a hydrophobic core, enabling us to address this question. A critical turn (turn 9/10) in the SLB was replaced with a segment with an intentional structural mismatch. Its crystal structure revealed a one-residue insertion into the central beta-strand (strand 9) of the SLB. This insertion triggered a surprisingly large-scale structural rearrangement: (i) the central strand (strand 9) was shifted by one residue, causing the strand to flip with respect to the adjacent beta-strands and thus completely disrupting the native side-chain contacts; (ii) the three-residue turn located on the opposite end of the beta-strand (turn 8/9) was pushed into its preceding beta-strand (strand 8); (iii) the register between strands 8 and 9 was shifted by three residues. Replacing the original sequence for turn 8/9 with a stronger turn motif restored the original strand register but still with a flipped beta-strand 9. The stability differences of these distinct structures were surprisingly small, consistent with an energy landscape where multiple low-energy states with different beta-sheet configurations exist. The observed conformations can be rationalized in terms of maximizing the number of backbone H-bonds. These results suggest that adjacent beta-strands "stick" through the use of factors that are not highly sequence specific and that beta-strands could slide back and forth relatively easily in the absence of external elements such as turns and tertiary packing.
authors.affiliation: Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Koki
authors.initials: K
authors.lastname: Makabe
authors.forename: Shude
authors.initials: S
authors.lastname: Yan
authors.forename: Valentina
authors.initials: V
authors.lastname: Tereshko
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Gawlak
authors.forename: Shohei
authors.initials: S
authors.lastname: Koide
mesh_terms: D000595:Amino Acid Sequence; D000954:Antigens, Surface / Q000737:chemistry* / Q000235:genetics; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000235:genetics; D001428:Bacterial Vaccines / Q000737:chemistry* / Q000235:genetics; D001898:Borrelia / Q000737:chemistry / Q000235:genetics; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008074:Lipoproteins / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013816:Thermodynamics
chemical_list: D000954:Antigens, Surface; D001425:Bacterial Outer Membrane Proteins; D001428:Bacterial Vaccines; D008074:Lipoproteins; C072045:OspA protein