HIV-1 is part of the lentiviral genus of retroviruses, a large and divers familiy of enveloped RNA viruses.
The hallmark of the Retroviridae family is its replicative strategy, which includes the reverse transcription of the virion RNA into linear double stranded DNA and its subsequent genomic integration. All retroviruses contain three major coding domains with information for virion proteins: gag, which directs the synthesis of internal virion proteins that form the matrix, the capsid, and the nucleoprotein structures; pol, which contains the information for the reverse transcriptase, integrase and protease enzymes; and env, from which the surface and transmembrane components of the viral envelope protein are derived. Simple retroviruses usually carry only this essential information, whereas complex retroviruses code for additional regulatory non-structural proteins derived from multi-spliced messages.
HIV-1 mature virions, which measure 100 nm in diameter, are spherical particles wrapped by a lipid bilayer. Their surface is spiked with trimers of the viral envelope glycoprotein (Env), which is made up by the two non-covalently attached gp120 and gp41 subunits resulting from the gp160 envelope precursor cleavage by a cellular furin-like protease (Freed and Martin, 2001). Virions are composed of a sub-membranal matrix shell and an innermost conical core formed by a paracrystalline assembly of capsid monomers. This latter structure contains the enzymatic machinery required for the early steps of the viral replication, namely the reverse transcriptase (RT) and integrase (IN), as well as two copies of the genomic RNA, each flanked by two cis-acting sequences partly repeated at both extremities. These, named LTRs (long terminal repeats), are required for reverse transcription, integration, gene expression and RNAs polyadenylation. Furthermore, the nucleocapsid (NC) protein associates with these RNAs through its potent RNA-binding activity.