Assembly of infectious herpes simplex virus type 1 virions in the absence of full-length VP22

VP22, the 301-amino-acid phosphoprotein product of the herpes simplex virus type 1 (HSV-1) U(L)49 gene, is incorporated into the tegument during virus assembly. We previously showed that highly modified forms of VP22 are rest ricted to infected cell nuclei (L. E. Pomeranz and J. A. Blaho, J. Virol. 7 3:6769-6781, 1999). VP22 packaged into infectious virions appears undermodi fied, and nuclear- and virion-associated forms are easily differentiated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (J, A. Blaho, C, Mitchell, and B. Roizman, J. Biol. Chem. 269:17401-17410, 1994). As VP22 p ackaging-associated undermodification is unique among HSV-1 tegument protei ns, we sought to determine the role of VP22 during viral replication. We no w show the following. (i) VP22 modification occurs in the absence of other viral factors in cell lines which stably express its gene. (ii) RF177, a re combinant HSV-1 strain generated for this study, synthesizes only the amino -terminal 212 amino acids of VP22 (Delta 212). (iii) Delta 212 localizes to the nucleus and incorporates into virions during RF177 infection of Vero c ells. Thus, the carboxy-terminal region is not required for nuclear localiz ation of VP22. (iv) RF177 synthesizes the tegument proteins VP13/14, VP16, and VHS (virus host shutoff) and incorporates them into infectious virions as efficiently as wild-type virus. However, (v) the loss of VP22 in RF177 v irus particles is compensated for by a redistribution of minor virion compo nents. (vi) Mature RF177 virions are identical to wild-type particles based on electron microscopic analyses. (vii) Single-step growth kinetics of RF1 77 in Vero cells are essentially identical to those of wild-type virus. (vi ii) RF177 plaque size is reduced by nearly 40% compared to wild-type virus. Based on these results, we conclude that VP22 is not required for tegument formation, virion assembly/maturation, or productive HSV-1 replication, wh ile the presence of full-length VP22 in the tegument is needed for efficien t virus spread in Vero cell monolayers.