Expression and packaging of SIV Vpx mutant molecular clones. SIV molecular clones lacking Vpx expression were complemented in trans with indicated Vpx mutants and Vpx WT by transient transfection of 293 T cells. MG132 was added 24 h post-transfection. Supernatants were collected at 48 h and virions were concentrated by ultracentrifugation through a 20% sucrose cushion. Vpx and CA expression were analyzed by immunoblot.