Structural basis for heme binding by the Shr protein from Streptococcus pyogenes

Streptococcus pyogenes causes a range of infectious diseases. In an era of increasing antibiotic resistance, new antimicrobial strategies targeting virulence factors, rather than essential survival mechanisms, are being explored. A key virulence factor in S. pyogenes is the bacterial iron acquisition system, because iron is essential but limited in the host due to sequestration by proteins like hemoglobin. The bacteria S. pyogenes possesses the Shr protein that acquires heme from host hemoglobin and transfers it to Shp, a membrane proximity protein. Shr comprises multiple domains, including two NEAr-Transporter (NEAT) domains that directly bind to heme. While structural information of NEAT domains from other bacteria are available, the structure of NEAT domains from Shr remains unknown. In this study, crystal structures of Linker-NEAT1 and NEAT2 domains were determined to 2.35 Å resolution and 2.66 Å resolution, respectively. Structural and mutational analyses revealed that methionine residues play a key role in heme binding, which seems to be a characteristic of heme-binding proteins from S. pyogenes, but not of NEAT domains from other gram-positive species. These findings enhance our understanding of heme acquisition in S. pyogenes and may guide novel therapeutic approaches.