The herpes simplex virus U(L)20 protein compensates for the differential disruption of exocytosis of virions and viral membrane glycoproteins associated with fragmentation of the Golgi apparatus

The Golgi apparatus is fragmented and dispersed in Vero cells but not in human 143TK- cells infected with wild-type herpes simplex virus 1. Moreover, a recombinant virus lacking the gene encoding the membrane protein U(L)20 (U(L)20- virus) accumulates in the space between the inner and outer nuclear membranes of Vero cells but is exported and spreads from cell to cell in 143TK- cell cultures. Here we report that in Vero cells infected with U(L)20- virus, the virion envelope glycoproteins were of the immature type, whereas the viral glycoproteins associated with cell membranes were fully processed up to the addition of sialic acid, a trans-Golgi function. Moreover, the amounts of viral glycoproteins accumulating in the plasma membranes were considerably smaller than those detected on the surface of Vero cells infected with wild-type virus. In contrast, the amounts of viral glycoproteins present on the plasma membranes of 143TK- cells infected with wild-type or U(L)20- virus were nearly identical. We conclude that (i) in Vero cells infected with U(L)20- virus the block in the export of virions is at the entry into the exocytic pathway, and a second block in the exocytosis of viral glycoproteins associated with cytoplasmic membranes is due to an impairment of transport beyond Golgi fragments containing trans-Golgi enzymes and not to a failure of the Golgi oligosaccharide-processing functions; (ii) these defects are manifested in cells in which the Golgi apparatus is fragmented; and (iii) the U(L)20 protein compensates for these defects by enabling transport to and from the fragmented Golgi apparatus.