br Results br Discussion The

Results
Discussion The twin-cysteine motif is an evolutionary conserved motif in all primate lentiviruses except HIV-1 and some strains of SIVcpz according to sequence analysis (Bohl et al., 2013). It is assumed that only those SIVcpz which have lost the twin-cysteines successfully evolved to HIV-1 through cross-species transmissions. It can be speculated that the less stable Env trimer, which has lost the twin-cysteines, may have the advantage of being able to adapt to the human immune system. This notion is supported because the unstable trimer of HIV-1 induces less neutralizing antibodies, but more non-neutralizing antibodies, when compared to the more stable Env trimers such as HIV-2, in which the infection elicits higher titers of autologous neutralizing LY364947 (Nabs) against the envelope glycoproteins (de Silva et al., 2012, Kong et al., 2012a, Kong et al., 2012b, Ozkaya Sahin et al., 2012). From our previous study on SIVmac239 and this study on HIV-2 ST and SIVagm, it is interesting to see that there are two distinct phenotypes that occur after removing the two-cysteines, which are from two different evolutionary lineages (Courgnaud et al., 2001): 1). The SIVmac239 and HIV-2 ST phenotype, in which all cleaved gp120 is secreted to the medium and there is no membrane associated gp120, belong to the same evolutionary lineage (Courgnaud et al., 2001, Gao et al., 1992); 2). In another lineage, SIVagm produces a different phenotype, in which gp160 is unprocessed, so the gp160 levels increase and no cleaved gp120 can be detected in the medium or lysate. This suggests that the twin-cysteines effect on the envelope trimer formation can be through gp120/gp41 association as well as gp160 processing. It will be interesting to investigate the twin-cysteine motif in more depth in the population of SIVcpz envelope proteins. The twin-cysteine motif is present in few SIVcpz species, but is largely absent from most published sequences. It is also important to note that SIVcpz causes a much more severe disease in its natural host than other SIVs, such as SIVagm and SIVsmm (Keele et al., 2009, Rudicell et al., 2010, Rudicell et al., 2011). SIVcpz infection results in CD4+ T cell depletion and studies suggest that naturally infected chimpanzees have a much higher mortality rate than their uninfected counterparts (Etienne et al., 2011, Terio et al., 2011). Based on sequence analysis, it seems like the twin-cysteine motif was lost within the SIVcpz population, so it will be interesting to see if the presence of this motif correlates with changes in pathogenesis. It will also be important in future studies to examine how the twin-cysteine motif is involved in stabilization of the envelope trimer since the structures of SIV or HIV-2 envelope are yet to be solved. According to our modeling, the twin-cysteines form a disulfide-bond that can stabilize this region in the V2 loop. It is also anticipated that this twin-cysteine loop can interact with the V1 loop, which is able to exert its effect for stabilization on the apex of the trimer. It seems possible that the alteration of the twin-cysteine motif may destabilize the whole envelope complex by disrupting the interaction of these two loops, causing a massive destabilization of the envelope complex. Interestingly, the SIV V1 loop has been found to be immunodominant, and a linear neutralizing epitope in the SIV V1 loop region has been identified (Jurkiewicz et al., 1997, Petry et al., 2000). Additionally, the interactions between V1 and V2 appear to be significant for virus genetic diversity in nature (Ondoa et al., 2001). Additionally, the lengths of the major loops (V1, V2 and V3) of HIV-1 are indispensable for envelope structural integrity and virus infection (Yuan et al., 2013), and their variation are also found to be involved in viral envelope function and disease progression (Curlin et al., 2011). The twin cysteines can form a disulfide bond to stabilize V2 loop as well as to increase the interactions with V1 loop (Fig. 8F). HIV-1 has lost these twin-cysteines which has decreased the stability of Env trimer, which may be part of the strategy for adapting to the stringent human immune system. This may help explain why HIV-1 is more successful than HIV-2 in humans from the respective cross-species transmissions.