Structural Basis for how Sialoglycan-binding Viridans Streptococci Accommodate Ligands that Exceed the Characterized Binding Site

During endocardial infections, viridans group streptococci use proteins containing siglec-like binding regions to engage sialic acid-capped <i>O-</i> GalNAc glycans on platelet glycoprotein GPIbα. Much past work used isolated di-, tri-, or tetrasaccharide partial ligands to interrogate this sialoglycan binding. Here, we report the 1.9 Å resolution crystal structure of the <i>Streptococcus gordonii</i> strain M99 siglec-like binding region bound to an L-serine-linked sialyl T antigen (sTa) trisaccharide. The structure demonstrates how trisaccharide extensions are accommodated, with implications for binding larger sialoglycan ligands.