P-C21 Molecular Mimicry of a Helical Region in Domain 1 of CD4 Facilitates Interdomain Stabilization of HIV-1 gp120

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The HIV-1 envelope spike displayed on the surface of infectious virions is a homotrimer of gp120:gp41 heterodimers. In its closed, pre-fusion form, the envelope trimer features a highly flexible and metastable conformation, which facilitates immune evasion, but it undergoes dramatic conformational changes upon binding to CD4 resulting in a stable, low- nergy post-fusion state. High-resolution structures of a soluble, disulfide-stabilized gp140 trimer (SOSIP) have provided important insights into the anatomy of the pre-fusion envelope. Here, we focused on a previously reported motif of 5 identical amino acids (SLWDQ) present both in CD4 (domain 1, DE loop) and in gp120 (inner domain, a1-helix), which also displays a remarkable structural homology being a right-handed helical structure in both proteins. Since the SLWDQ region of gp120 interacts intramolecularly with the b20-b21 CD4-binding loop in the outer domain, we hypothesized that the SLWDQ region of CD4 might replace its gp120 counterpart in the CD4-bound structure. Extensive molecular dynamic (MD) simulations corroborated this hypothesis. To experimentally verify the model, we introduced cysteine mutations in both Asp63 in the SLWDQ region of 4-domain soluble CD4 (sCD4) and Arg429 in the BG505-SOSIP.664 trimer, and the 2 mutated proteins were co-expressed in 293FS cells. Negative-staining electron microscopy (NSEM) and Western blot analyses under both reducing and non-reducing conditions documented the efficient formation of disulfide-bonded molecular complexes between the trimers and sCD4, thus validating the CD4 SLWDQ region as a contact surface for the gp120 b20-b21 loop. These results support the structural and functional mimicry between gp120 and CD4, which HIV-1 may exploit to optimize an intramolecular contact that is critical for stabilizing the inner-outer domain interaction in the pre-fusion envelope conformation.

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