At 24 h post-infection, cell culture supernatants from infected cells were cleared from cell debris and pelleted in an SW-60 rotor through a 20% sucrose cushion at 52000 rpm at 4uC for 2 h. The pellet was then resuspended in PBS buffer and mixed with SDS-PAGE sample buffer. To control the intracellular expression level, cell lysates were collected simultaneously. Samples were analyzed by SDS-PAGE and Western blotting using protein-specific antibodies as indicated.