Tools to interrogate glycoconjugate–protein interactions in the context of living cells are highly attractive for the identification of critically important functional binding partners of glycan-binding proteins. These interactions are challenging to study due to the low affinity and rapid dissociation rates of glycan–protein binding events. The use of photo-cross-linkers to capture glycan–protein interaction complexes has shown great promise for identifying binding partners involved in these interactions. Current methodologies use metabolic oligosaccharide engineering (MOE) to incorporate photo-cross-linking sugars. However, these MOE strategies are not amenable to all cell types and can result in low incorporation and cell-surface display of the photo-cross-linking probe, limiting their utility for studying many types of interactions. We describe here an exo-enzymatic strategy for selectively introducing photo-cross-linking probes into cell-surface glycoconjugates using the recombinant human sialyltransferase ST6GAL1 and a diazirine-linked CMP-Neu5Ac derivative. Probe introduction is highly efficient, amenable to different cell types, and resulted in improved cross-linking when compared to MOE. This exo-enzymatic labeling approach can selectively introduce the photo-cross-linking sugar onto specific glycan epitopes and subclasses by harnessing the specificity of the sialyltransferase employed, underscoring its potential as a tool to interrogate and identify glycoconjugate ligands for diverse glycan-binding proteins.

While bacterial natural products are a valuable source of therapeutics, the molecules produced by most biosynthetic gene clusters remain unknown. Tambjamine YP1, produced by Pseudoalteromonas tunicata, is partially derived from fatty acids siphoned from primary metabolism. A structurally similar tambjamine produced by Streptomyces, BE-18591, had not been linked to a gene cluster. Using enzymes putatively implicated in the construction of these two tambjamines, we used sequence similarity networks and gene knockout experiments to identify the biosynthetic gene cluster responsible for the production of tambjamine BE-18591 in Streptomyces albus. Despite the structural similarities between YP1 and BE-18591, the biosynthesis of the alkylamine tails of these molecules differs significantly, with the S. albus gene cluster putatively encoding a dedicated system for the construction of the fatty acid precursor to BE-18591. These different pathways in Pseudoalteromonas and Streptomyces suggest that evolutionary convergence is operative, with similar selective pressures leading to the emergence of structurally similar tambjamine natural products using different biosynthetic logic.

Tools to interrogate glycoconjugate-protein interactions in the context living cells are highly attractive for the identification of critically important functional binding partners of glycan-binding proteins. These interactions are challenging to interrogate due to low affinity and rapid dissociation rates of glycan-protein binding events. The use of photo-crosslinkers to capture glycan-protein interaction complexes has shown great promise for identifying binding partners involved in these interactions. Current methodologies use metabolic oligosaccharide engineering (MOE) to incorporate photo-crosslinking sugars. However, these MOE strategies are not amenable to all cell types and can result in low incorporation and cell-surface display of the photo-crosslinking probe, limiting their utility for studying many types of interactions. We describe here an exo-enzymatic strategy for selectively introducing photo-crosslinking probes into cell-surface glycoconjugates using the recombinant human sialyltransferase ST6GAL1 and a diazirine-linked CMP-Neu5Ac derivative. Probe introduction is highly efficient, amenable to different cell types and resulted in improved crosslinking when compared to MOE. This exo-enzymatic labeling approach can selectively introduce the photo-crosslinking sugar on to specific glycan epitopes and subclasses by harnessing the specificity of the sialyltransferase employed, underscoring its potential as a tool to interrogate and identify glycoconjugate ligands for diverse glycan-binding proteins.