For western blot analyses of intracellular proteins, cells were lysed by standard techniques as described previously [37] . For viral pellet analysis, cell supernatants were collected and viral particles concentrated 200 times by ultracentrifugation as previously described [38] . Protein expression was assessed by sodiumdodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blotting using the various antisera as indicated above. Membranes were incubated with a horse radish peroxidaseconjugated secondary antibody (GE Healthcare Life Sciences) and developed by chemiluminescence using Amersham ECL Plus Western Blotting Detection Reagents (GE Healthcare Life Sciences).