Bis-methionyl coordination in the crystal structure of the heme-binding domain of the streptococcal cell surface protein Shp.

摘要: Surface proteins Shr, Shp, and the ATP-binding cassette (ABC) transporter HtsABC are believed to make up machinery for heme uptake in Streptococcus pyogenes. Shp transfers its HtsA, lipoprotein component of HtsABC, providing only experimentally demonstrated example direct transfer from a surface protein an ABC Gram-positive bacteria. To understand structural basis this system, heme-binding domain (Shp(180)) was crystallized, structure determined resolution 2.1 A. Shp(180) exhibits immunoglobulin-like beta-sandwich fold that has been recently found other pathogenic bacterial cell proteins, suggesting mechanisms acquisition conserved. shows minimal amino acid sequence identity these reveals unique heme-iron coordination with axial ligands being two methionine residues same molecule. A negative electrostatic surrounding pocket may serve as docking interface more basic outer wall receptor Shr. The crystal exogenous, weakly bound hemins, which form large between molecules asymmetric unit. These "extra" hemins stacked pair similar observed previously free hemin dimers aqueous solution. propionates protein-bound coordinate iron atoms exogenous dimer, contributing stability interface. Gel filtration analytical ultracentrifugation studies indicate both full-length monomeric dilute