EVIDENCE FOR A MULTISTEP MECHANISM FOR CELL-CELL FUSION BY HERPES-SIMPLEX VIRUS WITH MUTATIONS IN THE SYN-3 LOCUS USING HEPARIN DERIVATIVESDURING FUSION FROM WITHIN

Addition of heparin-Na+ as well as related substances of high and inte rmediate MW (Arteparon and polyanion SP54) 3h after infection inhibit fusion from within (FFWI) induced by HSV strains with mutations in the syn 3 locus only. The concentration of heparin-Na+ required to inhibi t FFWI is 10-fold higher (1 mg/ml) than that needed to inhibit adsorpt ion. Instead of fusion, cell rounding is observed. The effect is readi ly reversible. A low MW heparin disaccharide is ineffective. Neomycin, at a concentration of 8 mM, inhibits FFWI induced by all HSV-1 but no t HSV-2 strains, whereas adsorption is inhibited at 3 mM. We conclude from our observations that cell-cell fusion (FFWI) induced by syn 3 lo cus mutants of HSV-1 depends on a multistep mechanism. One may be cons titued by pre-existing cell-cell connections or microfusions leading t o cell rounding, whereas another may be active using newly appearing c ell bridges during FFWI; also the three-dimensional structure of the c ell membrane may be of importance. Moreover, the molecular mechanisms of FFWI induced by mutations in the syn 3 locus compared to the other 5 syn loci should be different.