title: Metal binding is critical for the folding and function of laminin binding protein, Lmb of Streptococcus agalactiae.
abstract: Lmb is a 34 kDa laminin binding surface adhesin of Streptococcus agalactiae. The structure of Lmb reported by us recently has shown that it consists of a metal binding crevice, in which a zinc ion is coordinated to three highly conserved histidines. To elucidate the structural and functional significance of the metal ion in Lmb, these histidines have been mutated to alanine and single, double and triple mutants were generated. These mutations resulted in insolubility of the protein and revealed altered secondary and tertiary structures, as evidenced by circular dichroism and fluorescence spectroscopy studies. The mutations also significantly decreased the binding affinity of Lmb to laminin, implicating the role played by the metal binding residues in maintaining the correct conformation of the protein for its binding to laminin. A highly disordered loop, proposed to be crucial for metal acquisition in homologous structures, was deleted in Lmb by mutation (ΔLmb) and its crystal structure was solved at 2.6 Å. The ΔLmb structure was identical to the native Lmb structure with a bound zinc ion and exhibited laminin binding activity similar to wild type protein, suggesting that the loop might not have an important role in metal acquisition or adhesion in Lmb. Targeted mutations of histidine residues confirmed the importance of the zinc binding crevice for the structure and function of the Lmb adhesin.
authors.affiliation: Centre of Advanced Study in Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai, India.
authors.forename: Preethi
authors.initials: P
authors.lastname: Ragunathan
authors.forename: Divya
authors.initials: D
authors.lastname: Sridaran
authors.forename: Anja
authors.initials: A
authors.lastname: Weigel
authors.forename: Sarah
authors.initials: S
authors.lastname: Shabayek
authors.forename: Barbara
authors.initials: B
authors.lastname: Spellerberg
authors.forename: Karthe
authors.initials: K
authors.lastname: Ponnuraj
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D017353:Gene Deletion; D006801:Humans; D007797:Laminin / Q000737:chemistry / Q000378:metabolism*; D008670:Metals / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017510:Protein Folding; D017433:Protein Structure, Secondary; D012995:Solubility; D013050:Spectrometry, Fluorescence; D013292:Streptococcus agalactiae / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D007797:Laminin; D008670:Metals; D050505:Mutant Proteins; D015032:Zinc