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

## TL;DR

This study reveals how certain bacteria bind to complex sugar molecules on blood platelets, which helps them cause infections.

## Contribution

The paper provides a high-resolution crystal structure showing how a bacterial protein binds to a trisaccharide ligand.

## Key findings

- The 1.9 Å crystal structure shows how the bacterial protein accommodates trisaccharide extensions.
- The structure suggests how larger sialoglycan ligands may be bound by these bacteria.

## Abstract

During endocardial infections, viridans group streptococci use proteins containing siglec-like binding regions to engage sialic acid-capped
O-
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
Streptococcus gordonii
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.