The Siglec-like Serine-Rich Repeat (SRR) adhesins mediate bacterial attachment to mammalian hosts via sialoglycan receptors. Here, we combine structural, computational, biochemical, and phylogenetic approaches to elucidate the determinants of the sialoglycan-binding spectrum across the family of Siglec-like SRR adhesins. We further identified mutable positions that disproportionately affect sialoglycan selectivity, as demonstrated by increases in binding to alternative ligands of 2-to 3-orders of magnitude. Biologically, these studies highlight how bacteria nimbly modulate the receptor interaction during coevolution of host and pathogen. These studies additionally created binding proteins specific for sialyl-T antigen or 6S-sialyl Lewis X that can recognize glycosylation of human plasma proteins. The engineered binding proteins can facilitate the characterization of normal cellular glycan modifications or may be used as diagnostic tools in disease states with altered glycosylation.Significance: The ability of bacteria to bind selectively to host receptors underlies both commensalism and pathogenesis. Here, we identify the molecular basis for receptor selectivity in streptococci that bind to sialoglycan receptors. This revealed how to convert these adhesins into selective probes that measure tri-and tetrasacharides within the context of larger glycosylations. These probes that can be used in a laboratory with no specialized equipment and can be used to address biological questions relating to sialoglycan-dependent signaling and adhesion.

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Bacterial binding to host receptors underlies both commensalism and pathogenesis. Many streptococci adhere to protein-attached carbohydrates expressed on cell surfaces using Siglec-like binding regions (SLBRs). The precise glycan repertoire recognized may dictate whether the organism is a strict commensal versus a pathogen. However, it is currently not clear what drives receptor selectivity. Here, we use five representative SLBRs and identify regions of the receptor binding site that are hypervariable in sequence and structure. We show that these regions control the identity of the preferred carbohydrate ligand using chimeragenesis and single amino acid substitutions. We further evaluate how the identity of the preferred ligand affects the interaction with glycoprotein receptors in human saliva and plasma samples. As point mutations can change the preferred human receptor, these studies suggest how streptococci may adapt to changes in the environmental glycan repertoire.