It has been well established that human immunodeficiency viruses (HIV) are derived from simian immunodeficiency viruses (SIV) through cross-species transmission 
. The introduction of primate lentiviruses (PLV) into new host species can result in pathogenesis. Unlike SIV in nonhuman primates, HIV-1 frequently causes human immune system failure and leads to fatal acquired immunodeficiency syndrome (AIDS) 
. HIV-1 has infected more than 60 million people and caused the AIDS-related deaths of 25 million people globally (UNAIDS, Report on Globe AIDS Epidemic 2010). The basis for the differences between pathogenic HIV-1 infections in humans and the generally apathogenic SIV infections in African monkeys is not well understood. In the latter case, SIV and the host immune system apparently achieve a mutual balance. However, if the host loses the ability to control the virus, disease can result, as observed with HIV-1 in humans 
. Of interest, SIVcpz in chimpanzees, which represents the intermediate in PLV transmission from non-human primates to humans, can also exhibit pathogenicity in chimpanzees resembling that of HIV-1 in humans 
. Thus, SIV infection of monkeys, SIVcpz infection of chimpanzees, and HIV-1 infection of humans apparently represent examples of progressively poorer host immune system control of virus and increased pathogenicity.
The PLV, which include HIV-1, HIV-2 and SIV, are enveloped retroviruses. The trimeric envelope glycoprotein (Env) spikes on the virion surface are the only viral molecules making direct contacts with host cell receptors (CD4 and a chemokine receptor like CCR5). PLV Env evolution is driven by requirements to mediate host cell entry and to evade host-generated neutralizing antibodies. HIV and SIV Envs evade host immunity by employing mechanisms such as rapid alteration of surface loops, glycan shielding and presentation of multiple conformers, which may act as decoys 
. An understanding of HIV/SIV Env structure has relied on X-ray crystal structures of fragments of the gp120 and gp41 subunits and on low-resolution cryo-electron microscopic reconstructions 
. Unfortunately, the three-dimensional structure of the trimeric spike is still unknown, despite significant effort. However, cryo-electron tomography and single-particle electron microscopy approaches have yielded significant insights. Lower-resolution native Env trimer architectures on HIV-1 and SIV have been described 
. It has been suggested that the V1V2 regions are located at the membrane-distal apex of the Env spike 
. If the V1V2 regions are deleted, the SIV Env trimer can assume a more open and flexible structure 
. The recently reported HIV-1 Env trimer structure at 11-Å resolution shows that the V1, V2 and V3 variable regions of the gp120 exterior Env subunit interact near the trimer axis 
. This trimer-association domain (TAD) of gp120 potentially regulates trimer stability and other HIV/SIV Env phenotypes 
We are particularly interested in the participation of the V2 region in the trimer association domain (TAD) because so little is known about its structure and function. However, the V2 region is immunogenic, and in some cases serves as a target for broadly cross-reactive neutralizing antibodies 
. Recent data from antibody complexes with V2 peptides indicate that the V2 region of HIV-1 can potentially assume multiple conformations, depending on context 
Alignment of the PLV V2 sequences revealed the presence of two conserved cysteine residues in HIV-2 and SIV strains (). Because these two cysteines are always either present or absent as a pair during PLV evolution, we will refer to them as “twin cysteines”. This distinguishes them from other cysteine residues within the gp120 molecule, the disulfide-bonding pattern of which is well established 
. The twin cysteines in the gp120 V2 region of most SIV and HIV-2 strains are absent in all HIV-1 strains. We hypothesize that these twin cysteines may form a disulfide bond that plays a role in envelope trimer stabilization. Here, we provide data supporting this hypothesis, and show that the twin cysteines of SIVmac239 contribute significantly to stabilizing the non-covalent interaction of gp120 within the Env trimer.
Phylogeny of the gp120 V2 region in PLVs.