Glycoproteins M and N of human herpesvirus 8 form a complex and inhibit cell fusion

Glycoproteins M (gM) and N (gN) are well conserved across the herpesvirus family and their involvement in virus penetration and egress is well described, especially for alphaherpesviruses. Because there was no previous study on the homologues of human herpesvirus 8 glycoproteins M (gM8) and N (gN8), we analysed their biochemical and functional characteristics. We found that: (i) gM8 aggregated following heat treatment; (ii) gM8 was a virion component; (iii) gM8 and gN8 were N-glycosylated; (iv) gM8 formed a specific complex with gN8; and (v) gN8 was required for functional processing of gM8. Co-expression of gM8 and gN8 inhibited cell fusion induced either by a combination of herpes simplex virus type 1 glycoproteins or by Molony murine leukaemia virus envelope protein. These results indicate that, in addition to the similar biochemical properties, the fusion inhibition reported previously only for alphaherpesviruses is a function conserved in the gammaherpesvirus subfamily.