C-terminal domain of WSSV VP37 is responsible for shrimp haemocytes binding which can be inhibited by sulfated galactan

Viral envelope proteins play an important role in facilitating the attachment of viruses to the surface of host cells. Here, we investigated the binding of White Spot Syndrome Virus (WSSV) VP37 to haemocytes of whiteleg shrimp, Litopenaeus vannamei. Three versions of recombinant VP37 proteins, including full length VP37 (VP37_(1-281)), C-terminal domain VP37 (VP37_(111-281)) and C-terminal domain disrupted VP37 (VP37_(1-250)) were individually expressed and tested for their haemocytes binding ability. Through an ELISA-based binding assay, we found that VP37_(111-281) bound to shrimp haemocytes in a similar way to VP37_(1-281), while VP37_(1-250) exhibited a significantly weaker binding. This suggests that the C-terminal domain of VP37 is required for the binding of VP37 to shrimp haemocytes. Furthermore, we found that the binding of VP37 to shrimp haemocytes was impaired by pre-incubation of VP37 with sulfated galactan (SG), a sulfated polysaccharide derived from red seaweed (Gracilaria fisheri). Previously, it has been shown that a type of sulfated polysaccharide, heparin, is also present in L. vannamei. To investigate the role of heparin as a receptor for VP37, the binding of VP37 to porcine heparin, whose structure is similar to that found in L.vannamei, was investigated in a Surface Plasmon Resonance (SPR) system. The results showed that VP37 bound strongly to heparin with binding affinity (K_D) of 1.0 μM and the binding was significantly blocked by SG. These findings have lead us to propose that the attachment of WSSV might be mediated by the interaction between VP37 and a heparin-like molecule presented on the shrimp cells.