title: An intramolecular lock facilitates folding and stabilizes the tertiary structure of Streptococcus mutans adhesin P1.
abstract: The cariogenic bacterium Streptococcus mutans uses adhesin P1 to adhere to tooth surfaces, extracellular matrix components, and other bacteria. A composite model of P1 based on partial crystal structures revealed an unusual complex architecture in which the protein forms an elongated hybrid alpha/polyproline type II helical stalk by folding back on itself to display a globular head at the apex and a globular C-terminal region at the base. The structure of P1's N terminus and the nature of its critical interaction with the C-terminal region remained unknown, however. We have cocrystallized a stable complex of recombinant N- and C-terminal fragments and here describe a previously unidentified topological fold in which these widely discontinuous domains are intimately associated. The structure reveals that the N terminus forms a stabilizing scaffold by wrapping behind the base of P1's elongated stalk and physically "locking" it into place. The structure is stabilized through a highly favorable ΔG(solvation) on complex formation, along with extensive hydrogen bonding. We confirm the functional relevance of this intramolecular interaction using differential scanning calorimetry and circular dichroism to show that disruption of the proper spacing of residues 989-1001 impedes folding and diminishes stability of the full-length molecule, including the stalk. Our findings clarify previously unexplained functional and antigenic properties of P1.
authors.affiliation: Departments of Oral Biology and Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610.
authors.forename: Kyle P
authors.initials: KP
authors.lastname: Heim
authors.affiliation: Departments of Oral Biology and.
authors.forename: Paula J
authors.initials: PJ
authors.lastname: Crowley
authors.affiliation: Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610.
authors.forename: Joanna R
authors.initials: JR
authors.lastname: Long
authors.affiliation: Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610.
authors.forename: Shweta
authors.initials: S
authors.lastname: Kailasan
authors.affiliation: Biochemistry and Molecular Biology, University of Florida, Gainesville, FL 32610 rmckenna@ufl.edu jbrady@dental.ufl.edu.
authors.forename: Robert
authors.initials: R
authors.lastname: McKenna
authors.affiliation: Departments of Oral Biology and rmckenna@ufl.edu jbrady@dental.ufl.edu.
authors.forename: L Jeannine
authors.initials: LJ
authors.lastname: Brady
mesh_terms: D018829:Adhesins, Bacterial / Q000737:chemistry* / Q000235:genetics; D018360:Crystallography, X-Ray; D017510:Protein Folding; D055550:Protein Stability; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013295:Streptococcus mutans / Q000737:chemistry* / Q000235:genetics
chemical_list: D018829:Adhesins, Bacterial
keywords: Streptococcus; X-ray crystallography; adhesin; intramolecular lock; protein folding