The UL20 Gene of Herpes Simplex Virus 1 Encodes a Function Necessary for Viral Egress

A recombinant virus from which the start codon and 53% of the UL20 open reading frame had been deleted was constructed and characterized. We report the following: (i) The UL20- mutant formed small plaques in 143 tk- cells but failed to form plaques in Vero cells. Virus yields were approximately 10- to 100-fold lower than those of wild-type virus in all cell lines tested. (ii) Electron microscopic examination of Vero cells infected with the UL20- mutant revealed that enveloped and unenveloped capsids accumulated in the cytoplasm, possibly in the space between the inner and outer lamellae of the nuclear membrane, and that virtually no virus was present in the extracellular space. (iii) Glycoproteins B, C, D, E, H, and I recovered from lysates of cells infected with the UL20- mutant could not be differentiated from those present in lysates of cells infected with the wild-type parent virus with respect to the electrophoretic mobility of mature and precursor forms. (iv) Repair of the deleted sequences restored the wild-type phenotype. (v) The gene product of the UL20 gene was shown to be associated with cellular membranes and to possess characteristics of integral membrane proteins. We conclude that the UL20 gene encodes an integral membrane protein with a hitherto unrecognized function in that it enables the transit of virions to the extracellular space. The function of the UL20 gene product is complemented by some cell lines but not by Vero cells. The vesicles which serve to transport virions may have an origin different from those associated with transport of normal cellular proteins.