This thesis defines a novel role for the Herpes Simplex Virus (HSV) tegument protein, virion protein (VP) 11/12 as a modulator of host cell signalling. Studies aimed at examining infection induced lymphocyte inactivation, revealed that VP11/12 is tyrosine phosphorylated in three lymphocyte lineages (T cell, B cell and NK cell) following exposure to HSV-1 or HSV-2 infected fibroblasts. Tyrosine phosphorylation of VP11/12 was greater in lymphocytes compared to fibroblasts or epithelial cells and phosphorylation was enhanced by the lymphocyte specific Src family kinase (SFK) Lck during transfection- or infection-based assays. This suggested that VP11/12 is a substrate of Lck or a kinase activated by Lck. Lck is best known for initiating intracellular signalling downstream of the T cell receptor (TCR) and NK cell receptors. However, VP11/12 null HSV mutants retained the ability to block TCR signalling and NK cell cytotoxicity. Phosphorylation of VP11/12 occurred in the absence of any known Lck stimulus, like TCR ligation. Infection alone may activate Lck since Lck in infected Jurkat cells displayed features characteristic of activation: a reduced electrophoretic mobility in sodium dodecyl sulphate polyacrylamide gel and a marked increase in phosphorylation at the activation loop tyrosine (Y394). SFK substrates sometimes activate their cognate kinase through high affinity binding of the SFK Src homology (SH) 2 or SH3 domains. VP11/12 may serve this dual function since it interacts with Lck or Lck signalling complexes and is strictly required for Lck activation during infection. SFKs including Lck lie upstream of the canonical phosphoinositide 3-kinase (PI3K)-Akt pathway in signalling emanating from immune receptors, growth factor receptors and polyoma middle T antigen (MTAg). In HSV infection of Jurkat T cells and human embryonic lung fibroblasts, we find that VP11/12 interacts with PI3K either directly or indirectly and is required for infection induced activation of the PI3K-Akt signalling pathway. SFK activity is required for tyrosine phosphorylation of VP11/12, VP11/12-PI3K interactions, and Akt activation in infected fibroblasts. This data suggests that VP11/12 orchestrates signalling analogous to that of MTAg. In this model, VP11/12 activates SFKs to induce its own phosphorylation, subsequently allowing for interactions with PI3K and activation of Akt. / Virology

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