TheCrystal StructureofDecamericBslA RevealsTwoDistinct Structural Forms. From the data presented thus far, we suggest that the cap region of monomeric BslA is a random coil in aqueous solution with the hydrophobic side chains buried; at an interface the cap restructurestoformaβ-sheet,andself-assemblesintoa2Dlattice. Further supporting data for this model comes from the X-ray crystal structure (10): analysis reveals two substantially different capconfigurationsinthe decamericrepeatunit(Fig. 4A). Eight of thetensubunitsarepositionedwiththeircapsincloseproximityin a micelle-like arrangement. In these proteins, the cap regions are in a β-sheet configuration with the hydrophobic residues oriented
outward (Fig. 4B), creating the oily core of the micelle. The remaining two subunits (chains I and J) are farther away from the center of the decamer and the cap regions are in a random coil configuration with many of the hydrophobic side chains oriented away from the solvent (Fig. 4B). Taken together, our findings are consistent with chains A–H representing the interfacially bound form, and chains I and J the structure adopted in aqueous solution. This difference highlights the structural plasticity of the cap region, which undergoes substantial rearrangement in different solvent environments. The crystal structure of BslA indicates that the majority of the protein adopts a β-sheet structure; however, the region encompassing amino acids94–111forms a partially structured loop.We cannot rule out the possibility that this region also restructures at an interface, contributing to the increase in β-sheet content observedby CD, however systematicmutagenesisreveals that mutations in this region have no in vivo phenotypic consequences (T94A; K95M; D96N; T97A; L98A; N99A; A102M; L103A; R104M; L109A; N110S; N111S) (SI Appendix, Fig. S7). Thus, if this region restructures, it has no functional consequence with respect to hydrophobicity or self-assembly of BslA, processes that underpin successful biofilm assembly.
TheCrystal StructureofDecamericBslA RevealsTwoDistinct Structural Forms. From the data presented thus far, we suggest that the cap region of monomeric BslA is a random coil in aqueous solution with the hydrophobic side chains buried; at an interface the cap restructurestoformaβ-sheet,andself-assemblesintoa2Dlattice. Further supporting data for this model comes from the X-ray crystal structure (10): analysis reveals two substantially different capconfigurationsinthe decamericrepeatunit(Fig. 4A). Eight of thetensubunitsarepositionedwiththeircapsincloseproximityin a micelle-like arrangement. In these proteins, the cap regions are in a β-sheet configuration with the hydrophobic residues orientedoutward (Fig. 4B), creating the oily core of the micelle. The remaining two subunits (chains I and J) are farther away from the center of the decamer and the cap regions are in a random coil configuration with many of the hydrophobic side chains oriented away from the solvent (Fig. 4B). Taken together, our findings are consistent with chains A–H representing the interfacially bound form, and chains I and J the structure adopted in aqueous solution. This difference highlights the structural plasticity of the cap region, which undergoes substantial rearrangement in different solvent environments. The crystal structure of BslA indicates that the majority of the protein adopts a β-sheet structure; however, the region encompassing amino acids94–111forms a partially structured loop.We cannot rule out the possibility that this region also restructures at an interface, contributing to the increase in β-sheet content observedby CD, however systematicmutagenesisreveals that mutations in this region have no in vivo phenotypic consequences (T94A; K95M; D96N; T97A; L98A; N99A; A102M; L103A; R104M; L109A; N110S; N111S) (SI Appendix, Fig. S7). Thus, if this region restructures, it has no functional consequence with respect to hydrophobicity or self-assembly of BslA, processes that underpin successful biofilm assembly.
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