Terminal Epitope-Dependent Branch Preference of Siglecs Toward N-Glycans

Abstract

Siglecs are sialic acid–binding immunoglobulin-like lectins that play vital roles in immune cell signaling. Siglecs help the immune system distinguish between self and nonself through the recognition of glycan ligands. While the primary binding specificities of Siglecs are known to be divergent, their specificities for complex glycans remain unclear. Herein, we determined N-glycan binding profiles of a set of Siglecs by using a complex asymmetric N-glycan microarray. Our results showed that Siglecs had unique terminal epitope-dependent branch preference when recognizing asymmetric N-glycans. Specifically, human Siglec-3, -9, and -10 prefer the α1-3 branch when Siaα2-6Galβ1-4GlcNAc terminal epitope serves as the binding ligand but prefer the opposite α1-6 branch when Siaα2-3Galβ1-4GlcNAc epitope serves as the ligand. Interestingly, Siglec-10 exhibited dramatic binding divergence toward a pair of Neu5Ac-containing asymmetric N-glycan isomers, as well as their Neu5Gc-containing counterparts. This new information on complex glycan recognition by Siglecs provides insights into their biological roles and applications.

Systematic synthesis of bisected N-glycans and unique recognitions by glycan-binding proteins

“…Strikingly, bisected asymmetric N -glycan 16 and its non-bisected counterpart elicited highest binding signals for both Siglecs, whereas their positional isomers ( 16i and its non-bisected counterpart) showed no bindings. This result is consistent with our recent observation that Siglec-3 and -10 had a unique terminal epitope-dependent branch preference, 71 where both branches of compound 16 presented preferred terminal-epitope (6′SLN on the α1-3 branch, a less preferred ligand 3′SLN on the α1-6 branch). Another observation is that the bisecting GlcNAc can be tolerated by both Siglecs.…”

CarboGrove: a resource of glycan-binding specificities through analyzed glycan-array datasets from all platforms

“…The need for algorithms to discern the complex, fine specificities of glycan-binding proteins is clear, given the sensitivity of binding to minor differences in glycans such as the position of the epitope on N -glycan branches ( Li, Guan, et al. 2019a ; Wang et al. 2021 ).…”

Carbohydrate Sulfation As a Mechanism for Fine-Tuning Siglec Ligands

“…A clear glycan structural determinant directing recognition is the linkage between the sialic acid and the monosaccharide to which it is appended . For example, Siglec-2 (CD22) selectively recognizes α2–6 linked sialosides, while Siglec-3 (CD33) can bind both α2–3 and α2–6 linked sialosides. , The type of glycans to which the sialic acid is attached (e.g., N - or O -glycan, glycolipid), , underlying structural modifications (e.g., fucosylation and branching), , as well as glycan modifications (e.g., acetylation and sulfation) , also significantly impact the affinity of Siglecs for glycans. , Moreover, the protein context in which these glycans are presented has also been shown to play a role, for example, the density of presented glycans on the polypeptide backbone as in the case of tightly clustered O -glycans on mucins. , …”