Hiv env proteins
IntroductionHuman Immunodeficiency Virus (HIV) is a retrovirus/lentivirus that expresses a spike protein on its viral envelope (Env). "The envelope protein is synthesized as a precursor, gp160, which trimerizes and undergoes cleavage into two noncovalently associated fragments, the receptor-binding fragment gp120 and the fusion fragment gp41" [1]. Thus, Env is a heterodimer consisting of gp120 and gp41 and forms trimers on the surface of the viral membrane. As "the sole antigen on the virion surface," an understanding of Env (gp 120 & 41) and the conformational changes that occur in gp120 and gp41 during binding and fusion, respectively, is necessary in developing new fusion inhibitors and possible vaccines [1] [2]. The protein structure shown on the right is the gp120 portion of Env in its unbound trimer state [2].
Env StructureThe viral Env protein, as mentioned above, is composed of gp120 and gp41 that forms trimers on the viral membrane surface.  The image to the right, displays the undecorated Env trimer. Within the gp120 monomeric proteins are variable loops that undergo conformational changes that support binding or restrict binding of receptors such as CD4 or antibodies. Two important loops in this regard are the V1/V2 loops. The V1/V2 loops are located at the apex of the structure in the center of the three monomeric gp120 proteins. Depending on the gp120 conformation changes, the three gp41 transmembrane proteins, located beneath the gp120 surface proteins (located by the white arrow in F2.b), will undergo "irreversible refolding," bringing the viral membrane and host cell membrane closer together, consequently inducing fusion [1].
gp120 Bindinggp120 is responsible for the binding of HIV to CD4 cells. "Binding of gp120 to the primary receptor CD4 and coreceptor (for example, CCR5 and CXCR4) induces large conformational changes, which then trigger dissociation of gp120 and a cascade of refolding events in gp41" [1]. In the unliganded state, the V1/V2 loops are located at the center of the apex of the trimer and hold the trimer together. However, when CD4 binds to gp120, the V1/V2 loops undergo an outward rotational change that expose the central gp41 proteins at the base of the Env protein. gp120 is often complexed with CD4 and an antibody Fab (17b-Fab). "The 17b antibody has been shown to stabilize and lock gp120 in the CD4-bound conformation" [2]. "Residues in contact are concentrated in the span from 25 to 64 of CD4, but they are distributed over six segments of gp120" [3]. However, the most important gp120/CD4 interactions are between "Phe 43 and Arg 59 of CD4 [which] make multiple contacts centered on residues Asp 368, Glu 370 and Trp427 of gp120, which are all conserved among primate immunodeficiency viruses" [3]. 
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gp41gp41 is a transmembrane protein on the viral envelope of HIV, with its "C-terminal transmembrane segment inserted in the viral membrane" and its N-terminal outside the viral membrane beneath the gp120 proteins [1]. gp41 facilitates fusion of HIV with the host cell. There are 3 different conformations that gp41 can exist in: prefusion state, prehairpin intermediate, and post fusion state. In the prefusion state gp41 exists as the transmembrane segment mentioned above. Upon binding of gp120 to CD4, gp41 enters the prehairpin intermediate, where it extends into the host cell and the C-terminal segment remains in the viral envelope. Further rearrangement induces the post fusion state, where gp41 is completely enters the host cell, causing fusion of the membranes. gp41 potentially binds to neutralizing antibodies or non-neutralizing antibodies. "Neutralizing antibodies target a region on gp41 adjacent to the viral membrane, called the membrane-proximal external region (MPER)" represented by the red region [1]. However, non-neutralizing antibodies are more prevalent in infected individuals. "Cluster II antibodies recognize another immunodominant segment (residues 644–663) next to the MPER" [1]. These are non-neutralizing, however, because they bind to gp41 in the post fusion conformation, after the fusion is already complete.
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