Structural insights into SraP-mediated S. aureus adhesion to host cells

Staphylococcus aureus, a Gram-positive bacterium causes a number of devastating human diseases, such as infective endocarditis, osteomyelitis, septic arthritis and sepsis. S. aureus SraP, a surface-exposed serine-rich repeat glycoprotein (SRRP), is required for the pathogenesis of human infective endocarditis via its ligand-binding region (BR) adhering to human platelet. It remains unclear how SraP interacts with human host. Here we report the 2.05 Å crystal structure of the BR of SraP, revealing an extended rod-like architecture of four discrete modules. The N-terminal legume lectin-like module specifically binds to N-acetylneuraminic acid. The second module adopts a β-grasp fold similar to Ig-binding proteins, whereas the last two tandem repetitive modules resemble eukaryotic cadherins but differing in calcium coordination pattern. Small-angle X-ray scattering and molecular dynamic simulation indicated the three C-terminal modules function as a rigid stem to extend the N-terminal lectin module outward. Further structure-guided mutagenesis analyses showed that SraP binding to sialylated receptors promotes S. aureus adhesion to and invasion into host epithelial cells. Our findings have thus provided novel structural and functional insights into the SraP-mediated interaction of S. aureus with host epithelial cells.

Structure of pneumococcal GtfA reveals a novel prokaryotic O-GlcNAc transferase

Protein glycosylation is increasingly recognized as an important process for bacterial physiology and pathophysiology. Glycosylation of serine-rich repeat (SRR) glycoprotein PsrP is essential for the pathogenesis of Streptococcus pneumoniae, one of the most common human pathogens. This glycosylation process is initiated by a glycosyltransferase complex comprising two components, a core enzyme GtfA and a co-activator GtfB. Here we report the 2.0-Å crystal structure of GtfA in complex with GlcNAc and UDP. The structure possesses a core domain of GT-B fold and an unprecedented "add-on" domain of DUF1975, which adopts a β-meander structure. This novel DUF1975 domain is critical for the intact glycosyltransferase activity of GtfA–GtfB complex via mediating their self-recognition and the binding to the acceptor protein PsrP. The glycoproteomic analysis revealed a novel pattern of protein O-linked glycosylation at the serine residue cluster. The findings suggest that GtfA is a new glycosyltransferase and provide a structural basis for the future design of inhibitors against the biogenesis of bacterial SRRPs.