THE INTERACTION OF GLYCOPROTEIN-C OF HERPES-SIMPLEX VIRUS TYPE-1 AND TYPE-2 WITH THE ALTERNATIVE COMPLEMENT PATHWAY

Glycoprotein C (gC) of herpes simplex virus type 1 (HSV-1) or type 2 ( HSV-2) binds the human complement fragment C3b, but the two proteins d iffer in their ability to bind C3b on infected cell surfaces. In addit ion, gel, but not gC-2, accelerates the decay of the alternative pathw ay C3 convertase, thereby affecting later steps of the complement casc ade. Previously, we constructed linker insertion and deletion mutants of gC-1 and gC-2 and used transient transfection to express mutant pro teins in uninfected cells. In spite of the differences between gC-1 an d gC-2, C3b binding was localized to residues within the central porti on of both proteins, encompassing the first four cysteines. For gC-1, deletion mutants lacking amino acids 33 to 123 or 367 to 469 or lackin g both regions still bound C3b. We recombined these deleted forms of g C-1 into gC(-)39, an HSV-1 strain lacking the gC gene. The altered for ms of gC-1 were incorporated into virions, expressed on the surface of infected cells, and bound C3b. We used these proteins to investigate the structural basis for the inhibitory action of gC-1 on the compleme nt cascade. We found that gC-1 does not inhibit formation of the alter native pathway C3 convertase. This convertase is stabilized by the ser um protein properdin. Purified gC-1, but not gC-2, inhibits the bindin g of properdin to C3b, suggesting that this destabilizes the convertas e. The mutant lacking amino acids 367 to 449 was able to inhibit prope rdin binding to a limited extent when present at high concentrations, although it bound to C3b more weakly than wildtype gC. In contrast, th e protein lacking amino acids 33 to 123 was unable to inhibit properdi n binding to C3b. Thus, gC-1 contains two structural domains, one for C3b binding, residues 124 to 366, and another, residues 33 to 133, whi ch interferes with properdin binding to C3b. (C) 1994 Academic Press, Inc.