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HIV assembling on the surface of an

infected macrophage.

The phylogenetic tree of the SIV and

HIV

Assembly and release

The final step of the viral cycle, assembly of new HIV-1 virions,

begins at the plasma membrane of the host cell. The Env polyprotein

(gp160) goes through the endoplasmic reticulum and is transported to

the Golgi complex where it is cleaved by furin resulting in the two HIV

envelope glycoproteins, gp41 and gp120.[53]These are transported

to the plasma membrane of the host cell where gp41 anchors gp120to the membrane of the infected cell. The Gag (p55) and Gag-Pol

(p160) polyproteins also associate with the inner surface of the

plasma membrane along with the HIV genomic RNA as the forming

virion begins to bud from the host cell. The budded virion is still

immature as the gag polyproteins still need to be cleaved into the

actual matrix, capsid and nucleocapsid proteins. This cleavage is

mediated by the also packaged viral protease and can be inhibited by

antiretroviral drugs of the protease inhibitor class. The various

structural components then assemble to produce a mature HIV

virion.[54]Only mature virions are then able to infect another cell.

Genetic variability

Further information: Subtypes of HIV

HIV differs from many viruses in that it has very high genetic

variability. This diversity is a result of its fast replication cycle, with the

generation of about 1010virions every day, coupled with a high

mutation rate of approximately 3 x 105

per nucleotide base per cycleof replication and recombinogenic properties of reverse

transcriptase.[55][56][57]

This complex scenario leads to the generation of many variants of

HIV in a single infected patient in the course of one day.[55]This

variability is compounded when a single cell is simultaneously infected

by two or more different strains of HIV. When simultaneous infection

occurs, the genome of progeny virions may be composed of RNA

strands from two different strains. This hybrid virion then infects a new

cell where it undergoes replication. As this happens, the reversetranscriptase, by jumping back and forth between the two different RNA templates, will generate a newly

synthesized retroviral DNA sequence that is a recombinant between the two parental genomes. [55]This

recombination is most obvious when it occurs between subtypes.[55]

The closely related simian immunodeficiency virus (SIV) has evolved into many strains, classified by the natural

host species. SIV strains of the African green monkey (SIVagm) and sooty mangabey (SIVsmm) are thought to

have a long evolutionary history with their hosts. These hosts have adapted to the presence of the virus,[58]

which is present at high levels in the host's blood but evokes only a mild immune response, [59]does not cause

the development of simian AIDS,

[60]

and does not undergo the extensive mutation and recombination typical ofHIV infection in humans.[61]

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