CD4, located on the host cell membrane, is the main cellular receptor for the HIV protein gp120, which aids in mediating viral entry into target cells. The initial step in this cascade of events is the binding of viral gp120 protein to its host receptor, CD4. The key binding sites in CD4 for interaction with gp120 are located in the amino-terminal part of the CD4 molecule, distal to the transmembrane domain. The gp120 protein forms an oligomer (trimer) on the viral membrane with each gp120 protein containing variable domains (known as loops) and conservative domains. The V3 loop is also often obscured by gp120 glycosylation. Crystallization studies of CD4 suggest that the molecule has two immunoglobulin like domains important for the CD4/gp120 interaction, with one of the domains (D1) playing a more prominent role. Further studies suggest the Phe 43 and Arg 59 residues of CD4 play a major role in complex formation. Crystallization of gp120 shows that the polypeptide chain is folded into two major domains (an "inner" and "outer" domain with respect to the N and C termini), with the distal end of the “outer” domain containing the V3 loop. Studies of CD4 complexed with gp120 show that CD4 is bound to gp120 in a depression which is formed at the interface between the inner and outer domains. The complex itself is held together through van der Waals forces and hydrogen bonding.
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