Human immunodeficiency virus (HIV) is the human retrovirus associated with AIDS (acquired immune deficiency syndrome), and SIV its simian counterpart. Three main groups of primate lentivirus are known, designated Human immunodeficiency virus 1 (HIV-1), Human immunodeficiency virus 2 (HIV-2)/Simian immunodeficiency virus - mac (SIVMAC)/Simian immunodeficiency virus - sm (SIVSM) and Simian immunodeficiency virus - agm (SIVAGM). Simian immunodeficiency virus - mnd (SIVMND) has been suggested to represent a fourth distinct group PMID:2797181. These groups are believed to have diverged from a common ancestor long before the spread of AIDS in humans. Genetic variation in HIV-1 and HIV-2 has been studied extensively, and the nucleotide sequences reported for several strains PMID:2611042.

HIVs contain several virion-associated (auxiliary) proteins, such as Vpr and Vpx.

Vpx plays a role in nuclear translocation of the viral pre-integration complex (PIC) and is thus required for the virus to infect non-dividing cells. Vpr also plays a role in nuclear translocation of the PIC and may target specific host proteins for degradation by the 26S proteasome. It acts by associating with the cellular CUL4A-DDB1 E3 ligase complex through direct interaction with host VPRPB/DCAF-1. This would result in cell cycle arrest or apoptosis in infected cells, creating a favourable environment for maximising viral expression and production by rendering the HIV-1 LTR transcription more active.

The C-terminal domain (52-96) of Vpr seems to be involved in the most important functions of the protein, such as DNA interaction and apoptosis via interaction with the adenine nucleotide translocator. Vpr contains a Leu/Ile-rich domain (amino acids 60-81) in its C-terminal part, which is critical for dimerisation PMID:15571493. The C-terminal domain (amino acids 52-96) of Vpr was shown to be involved in cell cycle arrest, to bind the nucleocapsid protein NCp7, and to interact with HIV-1 RNA. Furthermore, this domain is thought to promote nuclear provirus transfer.

INTERPRO:IPR000012,DOI:10.1006/jmbi.1998.2381,DOI:10.1042/BJ20041759