Structural Basis for Sialoglycan Binding by the Streptococcus sanguinis SrpA Adhesin

Area of Science:

• Microbiology
• Structural Biology
• Infectious Diseases

Background:

• Streptococcus sanguinis is a primary cause of infective endocarditis, a severe cardiovascular infection.
• Bacterial attachment to host platelets is a critical step in infective endocarditis pathogenesis.
• The serine-rich repeat adhesin SrpA in S. sanguinis is implicated in platelet binding and virulence.

Purpose of the Study:

• To elucidate the structural basis of SrpA-mediated platelet interaction.
• To determine the crystal structure of the putative binding region of SrpA (SrpABR).
• To identify key residues and motifs involved in ligand binding.

Main Methods:

• X-ray crystallography was used to determine the structure of SrpABR.
• Structures were solved for both unligliganded SrpABR and its complex with a synthetic disaccharide ligand.
• Resolution achieved was 1.8 Å for the unliganded form and 2.0 Å for the complex.

Main Results:

• The crystal structure of the SrpA binding region (SrpABR) was determined.
• A conserved Thr-Arg motif was identified, crucial for orienting sialic acid and mediating platelet binding.
• Structural analysis suggests sequence variations in related adhesins modulate quaternary/tertiary structure and ligand-binding.

Conclusions:

• The Thr-Arg motif in SrpA is essential for binding to sialic acid on host platelets.
• Structural insights into SrpA provide a basis for understanding S. sanguinis pathogenesis.
• Modifications in related adhesins likely influence their interaction with host factors.