Crystal structure of octocoral lectin SLL-2 complexed with Forssman antigen tetrasaccharide

Abstract

A symbiosis-related lectin, SLL-2, from the octocoral Sinularia lochmodes, distributes densely on the cell surface of microalgae, Symbiodinium sp., an endosymbiotic dinoflagellate of the coral, and is also shown to be a chemical cue that transforms dinoflagellates into a non-motile (coccoid) symbiotic state. SLL-2 binds to the sugar chain of the molecule similar to Forssman antigen pentasaccharide (GalNAcα1-3GalNAcβ1-3 Galα1-4 Galβ1-4Glc) on the surface of microalgae with high affinity. Here we report the crystal structure of the complex between SLL-2 and Forssman antigen tetrasaccharide (GalNAcα1-3GalNAcβ1-3 Galα1-4 Galβ) at 3.4 Å resolution. In an asymmetric unit of the crystal, there are two hexameric molecules with totally twelve sugar recognition sites. At the nine in twelve sites, the first and second saccharides of the Forssman antigen tetrasaccharide bind directly to galactopyranoside binding site of SLL-2, whereas the third and fourth saccharides have no interaction with the SLL-2 hexameric molecule that binds the first saccharide. The sugar chain bends at α-1,4-glycosidic linkage between the third and fourth saccharides toward the position that we defined as a pyranoside binding site in the crystal structure of the complex between SLL-2 and GalNAc. The structure allowed us to suggest a possible binding mode of the Forssman antigen pentasaccharide to SLL-2. These observations support our hypothesis that the binding of SLL-2 to the cell surface sugars of zooxanthella in a unique manner might trigger some physiological changes of the cell to adapt symbiosis with the host coral.

Structure and assembly pattern of a freshwater short-tailed cyanophage Pam1

“…Furthermore, given that the distal C-terminal b-sandwich motifs of phage tailspikes are responsible for recognition of host extracellular polysaccharides (Muller et al, 2008), it is also possible that the cement proteins of Pam1 may have similar activity. Notably, the carbohydrate-binding proteins CBM36 (Jamal-Talabani et al, 2004) (PDB: 1ux7) and SLL-2 (Kita et al, 2017) (PDB: 5x4a), which are most similar in structure to the cement b-sandwich and the tailspike b-sandwich, respectively, bind to carbohydrates via acidic patches on the exposed loops. Structural analysis also revealed acidic patches on the outermost loops of both the cement b sandwich and the tailspike b sandwich.…”

A novel and highly specific Forssman antigen-binding protein from sheep polyomavirus

“…This feature distinguishes VP1 from many other Forssman glycan-binding proteins such as Galectin-9, Helix pomatia agglutinin (HPA), Sinularia lochmodes lectin-2, and Dolichos biflorus agglutinin, where recognition is based on the subterminal or terminal GalNAc (see Fig. S6), resulting in a much broader glycan binding capacity [64][65][66][67] .…”

A GM1b/asialo‐GM1 oligosaccharide‐binding R‐type lectin from purplish bifurcate mussels Mytilisepta virgata and its effect on MAP kinases

“…During this functional diversification, marine invertebrates developed an unusually large number of lectins, many having convergent structures that facilitate binding to specific glycan structures exposed on the surface of target cells. This combination of functional divergence and structural convergence has resulted in many unique sequences and unusual glycan-binding specificities among lectins isolated from marine invertebrates [7][8][9][10][11].…”