Herpes simplex virus type 1 glycoprotein gM and the membrane associated protein UL11 are required for virus-induced cell fusion and efficient virus entry

HSV-1 facilitates virus entry into cells and cell-to-cell spread by mediating fusion of the viral envelope with cellular membranes and fusion of adjacent cellular membranes. Although virus strains isolated from herpetic lesions cause limited cell fusion in cell culture, clinical herpetic lesions typically contain large syncytia, underscoring the importance of cell-to-cell fusion in virus spread in infected tissues. Certain mutations in gB, gK, UL20 and other viral genes, drastically enhance virus-induced cell fusion in vitro and in vivo. Recent work has suggested that gB is the sole fusogenic glycoprotein, which is regulated by interactions with viral glycoproteins gD, gH/gL, gK, membrane protein UL20 and cellular receptors. Recombinant viruses were constructed to abolish either gM or UL11 expression in the presence of strong syncytial mutations in either gB or gK. Virus-induced cell fusion caused by deletion of the carboxyl terminal 28 amino acids of gB, or the dominant syncytial mutation in gK (Ala-to-Val at amino acid 40), was drastically reduced in the absence of gM. Similarly, syncytial mutations in either gB or gK did not cause cell fusion in the absence of UL11. Neither the gM nor UL11 gene deletions substantially affected gB, gC, gD, gE and gH glycoprotein synthesis and expression on infected cell surfaces. Two-way immunoprecipitation experiments revealed that the membrane protein UL20, which is found as a protein complex with gK, interacted with gM, while gM did not interact with other viral glycoproteins. Viruses produced in the absence of gM or UL11 entered into cells slower than their parental wild-type virus. Moreover, the mouse eye model study, revealed that although the UL11-null virus replicated less efficiently in mouse corneas, it infected ganglionic neurons with greater efficiency than either the gM or gE-null viruses. Collectively, these results indicate that gM and UL11 are required for efficient membrane fusion events during virus entry and virus spread in vivo as well as in vitro, and also that gK plays the most important role in corneal and ganglionic infection in the mouse eye model followed by gM, gE, and UL11 in a descending order of importance relative to the wild-type virus.