Genetic diversity of the highly variable V1 region interferes with Human Immunodeficiency Virus type 1 envelope functionality

BACKGROUND: The HIV envelope (Env) promotes viral entry in the host cell. During this process, Env undergoes several conformational changes to ensure its function. At the same time, the gp120 component of Env is the protein of the virus presenting the largest genetic diversity. Understanding how the virus maintains the balance between the competing requirements for maintenance of functionality and antigenic variation of this protein is central for the comprehension of its strategies of evolution and can highlight vulnerable aspects of its replication cycle. We focused on the variable domains V1 and V2 of the HIV-1 gp120 that are involved in conformational changes and are critical for viral escape from antibody neutralization. RESULTS: Despite the extensive sequence diversity found in the epidemic for these regions and their location on the external face of the protein, we observed that replacing V1V2 of one primary isolate with that of another severely interferes with Env functionality in more than half of the cases studied. Similar results were obtained for intra- and intersubtype chimeras. These observations are indicative of an interference of genetic diversity in these regions with Env functionality. Therefore, despite the extensive sequence diversity that characterizes these regions in the epidemic, our results show that functional constraints seem to limit their genetic variation. Defects in the V1V2 chimeras were not relieved by the insertion of the V3 region from the same isolate, suggesting that the decrease in functionality is not due to perturbation of potential coevolution networks between V1V2 and V3. Within the V1V2 domain, the sequence of the hypervariable loop of the V1 domain seems to be crucial for the functionality of the protein. CONCLUSIONS: Besides the well-documented role of V1V2 in the interplay with the immune response, this work shows that V1 is also involved in the selection of functional envelopes. By documenting a compromise between the opposing forces of sequence diversification and retention of functionality, these observations improve our understanding of the evolutionary trajectories of the HIV-1 envelope gene.