gp13 (EHV-gC): a complement receptor induced by equine herpesviruses.

Equine herpesviruses type 1 (EHV-1) and type 4 (EHV-4) induce a complement receptor protein on the surface of infected cells capable of binding to the third component of complement (C3). The protein mediating the binding to the C3 component of complement was identified as glycoprotein 13 (gp13, EHV-gC), as expression of the cloned viral gene under the control of a CMV promoter induced C3 binding activity at the transfected cell surface. Comparable to glycoprotein C (gC) from herpes simplex virus type 1 (HSV-1-gC), glycoprotein III from pseudorabiesvirus (gIII, PRV-gC) and bovine herpesvirus-1 (gIII, BHV-1-gC), gp13 derived from EHV-infected cell lysates bound to C3 fixed to solid phase, showing preferential binding to the appropriate host complement component. Similar to wild-type isolates, a highly attenuated vaccine EHV-1 strain also displayed complement receptor activity despite apparent differences of the gp13 gene in restriction enzyme digest pattern and reactivity with monoclonal antibodies. In addition, other structural proteins were altered in the vaccine strain as compared to wild-type strains, which might contribute to its attenuated phenotype. In contrast to the situation observed with HSV-1-gC, the interaction of gp13 (EHV-gC) with horse complement was not inhibited by polyanionic substances like heparin or dextran sulfate. These results suggest structural differences in the particular binding mechanism of the respective viral envelope proteins.