Characterization of domains of herpes simplex virus type 1 glycoprotein E involved in Fc binding activity for immunoglobulin G aggregates.

Herpes simplex virus type 1 glycoproteins gE and gI form receptors for the Fc domain of immunoglobulin G (IgG) which are expressed on the surface of infected cells and on the virion envelope and which protect the virus from immune attack. Glycoprotein gE-1 is a low-affinity Fc receptor (FcR) that binds IgG aggregates, while gE-1 and gI-1 form a complex which serves as a higher-affinity FcR capable of binding IgG monomers. In this study, we describe two approaches used to map an Fc binding domain on gE-1 for IgG aggregates. First, we constructed nine plasmids encoding gE-1/gD-1 fusions proteins, each containing a large gE-1 peptide inserted into the ectodomain of gD-1. Fusion proteins were tested for FcR activity with IgG-sensitized erythrocytes in a rosetting assay. Three of the fusion proteins containing overlapping gE-1 peptides demonstrated FcR activity; the smallest peptide that retained Fc binding activity includes gE-1 amino acids 183 to 402. These results indicate that an Fc binding domain is located between gE-1 amino acids 183 and 402. To more precisely map the Fc binding domain, we tested a panel of 21 gE-1 linker insertion mutants. Ten mutants with insertions between gE-1 amino acids 235 and 380 failed to bind IgG-sensitized erythrocytes, while each of the remaining mutants demonstrated wild-type Fc binding activity. Taken together, these results indicate that the region of gE-1 between amino acids 235 and 380 forms an FcR domain. A computer-assisted analysis of the amino acid sequence of gE-1 demonstrates an immunoglobulin-like domain contained within this region (residues 322 to 359) which shares homology with mammalian FcRs.