How can human and simian immunodeficiency viruses utilize chemokine receptors as their coreceptors?

Several chemokine receptors (CKRs) act as coreceptors of human immunodeficiency virus type 1 (HIV-1), type 2 (HIV-2) and simian immunodeficiency virus (SIV). These CKRs interact with the V3 domain of the envelope (env) protein of HIV/SIV. In this study, we found that the amino acid sequences of two chemokines (SDF-1beta and RANTES), whose receptors (CXCR4 and CCR5) act as major coreceptors for HIV-1, HIV-2 or SIV, showed statistically significant similarity to those of the region containing the third variable (V3) and the third conserved (C3) domains (the V3--C3 domain) of the env protein of HIV-1 and HIV-2. We made a multiple alignment of amino acid sequences for 24 chemokines and the region encompassing the second conserved (C2), V3 and C3 domains (the C2--V3--C3 region) of 10 strains of HIV/SIV. Surprisingly, the hydropathic profile and several important amino acids for protein conformation, such as cysteine and tryptophan, are remarkably conserved between chemokines and the V3--C3 region of HIV/SIV. Moreover, hydrophobic amino acids, such as leucine, isoleucine and valine, are found to be clustered both in the amino-terminal region of chemokines and the C2 domain of HIV/SIV. Thus, chemokines have significantly similar profiles of amino acid properties to those of the C2--V3--C3 region of the env protein of HIV/SIV. These findings raise a hypothesis that chemokines and the C2--V3--C3 region have a common origin. Namely, the HIV/SIV ancestor incorporated a chemokine gene into its env gene. The captured chemokine gene has rapidly diverged by frequent mutations specific to the retroviral genome, and thereby obtained the ability to interact with various CKRs in a short period of time. This paper proposes that the capture of a ligand gene of the host cells into the viral genome may be one of the important mechanisms of viral evolution to expand its host range and generate new viral species.