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Interactions of HIV-1 Envelope Protein gp120 with the Cellular Receptor Protein CD4 and the CCR5 Co-Receptor Mimic 412d Antibody

James Beckett '11 and Maggie Taylor '11


I. Biological Motivation

The human immunodeficiency virus (HIV) has recently become one of the most studied viruses by medical researchers. These viruses can establish a lifelong infection of a host’s T-cells, leading to acquired immunodeficiency syndrome (AIDS), and eventually causing death of the individual. 

Type 1 HIV (HIV-1) contains the envelope glycoprotein gp120 which recognizes the host cell receptors CD4 and the co-receptors CCR5 or CXCR4. Once these proteins have interacted, HIV enters the cell by fusing the viral and cellular membranes. The envelope protein is required for the positioning and timing of this fusion (Kwong, et al, 1998).

The envelope protein gp120 contains 25 beta-strands, 5 alpha-helices, and 10 defined loop segments. Two conserved regions of gp120 provide most of the interactions with the T-cell co-receptor proteins: the bridging sheet and the V3 loop. The bridging sheet consists of beta sheets 2, 3, 20, and 21 . The V3 loop contains a conserved base, a flexible stem, and a beta-hairpin tip, providing a second set of interactions (Huang, et al, 2007).

Invasion of HIV-1 into the host cell first requires the binding of the HIV-1 envelope protein gp120 to the cellular receptor protein CD4 , orienting the viral spike, stabilizing the bridging sheet, and exposing the V3 loop of gp120.  This new orientation readily allows for viral interactions with the cellular co-receptor CCR5 (Huang, et al, 2007).

Genetic therapies disrupting the CCR5 protein are promising avenues of research for developing HIV resistance.  In 2006, a man with AIDS was treated for leukemia with a bone marrow transplant of stem cells from a person who naturally had a mutant, non-functioning version of the CCR5 protein.  This mutant CCR5 does not facilitate HIV-1 entry into cells.  The recipient of the transplant has shown no signs of HIV since then, and has been proclaimed “functionally” cured by the American Foundation for AIDS Research.  Many reservations exist about the treatment, but it has been proclaimed a proof of concept. 

II. In Vivo Binding Interactions

The CCR5 co-receptor is a seven-transmembrane protein  involved in chemokine signaling.  Once the HIV-1 gp120 glycoprotein binds to CD4, it will preferentially bind to residues contained in the N terminus (amino acid residues 2-15) and the second extracellular loop of CCR5. The N-terminus of CCR5 approaches from the same face of gp120 as does CD4 but binds to an orthogonal surface at the intersection of the bridging sheet and the V3 loop   (Huang, et al, 2005). The N-terminal interaction of the co-receptor with HIV-1 requires an unusual post-translational modification, o-sulfonation of tyrosine, creating sulfotyrosines (Tys) (Farzan, et al, 1999). On CCR5, tyrosines at residues 3, 10, 14, and 15 may be o-sulfonated, but Tys's at residues 10 and 14 are sufficient to facilitate interactions with gp120.

III. Crystal Structure 

Huang, et al were not able to crystallize the CCR5 protein with the
-complexed HIV-1 gp120 . Instead, an antigen- binding fragment (FAB) of the antibody 412d was crystallized with gp120 and CD4.

The light and heavy chains of 412d contain binding domains almost identical to those of CCR5. When bound, the sulfotyrosines of 412d very closely mimic the sulfotyrosines in the N terminus. 
Tys100c provides most of the interactions with residues in the V3 loop, acting as a replacement for Tys 14 from CCR5, while Tys100 mimics Tys10 . These two sulfotyrosines fulfill most of the major interactions between gp120 and its co-receptor protein CCR5, making 412d and excellent mimic for researchers to study (Huang, et al, 2007).

IV. 412d Interactions with gp120

The interactions between CCR5 and gp120 are mimicked by residues contained in the heavy chain of 412d. Hydrophobic residues of the second complementarity-determining region of the heavy chain (CDR H2) interact with the hydrophobic bridging sheet of gp120  .

In the second interaction, the acidic CDR H3 of the 412d heavy chain binds to basic residues on the gp120 surface. Two  sulfotyrosines (Tys100 and Tys100c) contribute the most essential binding interactions. Tys100  is mostly exposed, with its aromatic ring making pi-cation interactions with Arg327 of gp120 and its sulfate group making peripheral electrostatic interactions.  Tys100c uses its sulfate group for most of its interactions with gp120. Hydrogen bonding between the sulfate group of Tys100c and Thr303, Asn300, and Asn302 of gp120 are the strongest bonds. The sulfate can also make a salt bridge to Arg298 (Huang, et al, 2007).

V. CD4 Interactions with gp120

Direct interatomic contacts are made between 22
CD4  residues and 26 gp120 amino-acid residues. This represents 219 van der Waals forces and 12 hydrogen bonds. The most important interactions occurs between Phe 43 and Arg59 of CD4 with Asp368, Glu370, and Trp427 of gp120, which are conserved among all primate immunodeficiency viruses. Phe
43 alone accounts for 23 % of the total CD4-gp120 interaction.   
Phe43 is buried in a hydrophobic cavity and interacts with Trp112, Val255, Thr257, Asp368, Trp427
Ile 371, Glu370, Gly473 . Arg59 interacts with Asp368  . The carboxylate group of Asp368 makes two very strong hydrogen-bonds with the Arg59 residue , while the strength of the Phe 43 residue comes from its ability to stack between carboxylate groups and contact other residues hydrophobically. These interactions provide the strength to make a connection between the glycoprotein and the initial receptor protein (Kwong, et al, 1998).

Upon binding, CD4 will initiate an energetically-favorable conformational change in the structure of gp120. The binding of
Phe43 to the hydrophobic cavity of gp120 helps to form a more hydrophobic core of the protein. This  conformation stabilizes  beta-strands 2, 3, 20, and 21 into the bridging-sheet conformation. Additionally, this binding stabilizes the V3 loop seen in the crystal, positioning it to bind with the co-receptor CCR5. This sequential binding is necessary for HIV-1 entry into the host cell  (Kwong, et al, 1998).

VI. References

Farzan, M.,  T. Mirzabekov, P. Kolchinsky, R. Wyatt, M. Cayabyab, N. P. Gerard, C. Gerard, J. Sodrocki, H. Choe. 1999. Tyrosine sulfation of the amino terminus of CCR5 facilitates HIV-1 entry. Cell 96: 667-676.

Huang, Chih-chin. Min Tang, Mei-Yun Zhang, Shahzad Majeed, Elizabeth Montabana, Robyn L. Stanfield, Dimiter S. Dimitrov, Bette Korber, Joseph Sodroski, Ian A. Wilson, Richard Wyatt, Peter D. Kwong. 2005. Structure of a V3-Containing HIV-1 gp120 Core. Science 310: 1025-1028.

Huang, Chih-chin, Son N. Lam, Priyamvada Acharya, Min Tang, Shi-Hua Ziang, Syed Shahzad-ul Hussan, Robyn L. Stanfield, James Robinson, Joseph Sodroski, Ian A. Wilson, Richard Wyatt, Carole A. Bewley, and Peter D. Kwong. 2007. Structures of the CCR5 N Terminus and of a Tyrosine-Sulfated Antibody with HIV-1 gp120 and CD4.
Science 331:1930-1993.

Kwong, Peter D, Richard Wyatt, James Robinson, Raymond W. Sweet, Joseph Sodroski, and Wayne A. Hendrickson. 1998. Structure of an HIV gp120 envelope glycoprotein in complex with the CD4 receptor and a neutralizing human antibody. Nature 393:648-659.

Rizzuto, Carlo D, Richard Wyatt, Nivia Hernandez-Ramos, Ying Sun, Peter D. Kwong, Wayne A. Hendrickson, and Joseph Sodroski. 1998. A Conserved HIV gp120 Glycoprotein Structure Involved in Chemokine Receptor Binding. Science 280: 1949-1953. 

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