The Simian virus 40 late viral protein VP4 disrupts the nuclear envelope for viral release.

Simian virus 40 (SV40) appears to initiate cell lysis by expressing the late viral protein VP4 at the end of infection to aid in virus dissemination. To investigate the contribution of VP4 to cell lysis, VP4 was expressed in mammalian cells where it was predominantly observed along the nuclear periphery. The integrity of the nuclear envelope was compromised in these cells, resulting in the mislocalization of a soluble nuclear marker. Using assays that involved the cellular expression of VP4 or the treatment of cells with purified VP4, we found that the central hydrophobic domain and a proximal C-terminal nuclear localization signal of VP4 were required for (i) cytolysis associated with prolonged expression; (ii) nuclear envelope accumulation; and (iii) disruption of the nuclear, red blood cell, or host cell membranes. Furthermore, a conserved proline within the hydrophobic domain was required for membrane perforation, suggesting that this residue was crucial for VP4 cytolytic activity. These results indicate that VP4 forms pores in the nuclear membrane leading to lysis and virus release.