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Localization of Viral protein X in Simian Immunodeficiency Virus Macaque Strain and Analysis of its Packaging Requirements

Vladimir Liska^1,

¹ Unité INSERM 74 and Institut de Virologie de la Faculté de Médecine, Université Louis Pasteur, 3 rue Koeberlé, 67000 Strasbourg
and² Transgène, 11 rue de Molsheim, 67082 Strasbourg, France

Simian immunodeficiency virus (SIV) and human immunodeficiency virus type 2 (HIV-2) encode the accessory viral protein X (Vpx) known to be incorporated into virions in amounts comparable to those of the Gag proteins. The localization of Vpx within SIV_mac-infected HUT-78 cells and SIV_mac virions was studied by immunoelectron microscopy. Vpx appeared to be associated with extracellular virions as well as budding viral particles at the surface of infected cells. Immunolabelling of purified viral cores suggested that Vpx was a component of the amorphous material surrounding the core structure. Furthermore, a detergent insoluble fraction containing SIV core proteins was devoid of Vpx. To investigate the protein requirement for packaging of Vpx, BHK-21 cells were co-infected with vaccinia virus recombinants encoding Vpx and other SIV proteins able to assemble into virus-like particles. Analysis by immunoprecipitation of the extracellular particulate material as well as immunoelectron microscopy demonstrated that co-expression of Vpx with the Pr56^gag polyprotein was sufficient for the formation of pseudo-virions containing Vpx. Virus-like particles that appeared upon expression of p16^gag did not contain Vpx. The results suggest that Vpx is packaged into viral particles through its binding to the Gag polyprotein. The precise positioning of Vpx within the space separating the viral envelope from the core structure is postulated to result from the reorganization of viral proteins that occurs upon Gag polyprotein cleavage and budding.

Present address: Laboratory of Molecular Virology and Carcinogenesis, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland 21702-1201, U.S.A.

Received 21 February 1994; accepted 21 June 1994.

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