The HIV-1 envelope gp120 glycoprotein is made up of three autonomously folding domains that adopt different conformations in response to ligand binding. During HIV entry, the CD4 receptor engages a ~14 A deep cavity that penetrates the hydrophobic core of gp120 at the intersection of these three domains. Mutagenesis studies have demonstrated the importance of residues lining this cavity for virus entry. Here we use a set of CD4-mimetic miniproteins with different inserts into the Phe 43 cavity, to probe the structure of the cavity and decipher its role in ligand binding and neutralization. Through virological, biochemical and structural analyses we show that optimal filling of the Phe 43 cavity with a flexible ligand can lead to high affinity binding and broad and potent neutralization. CD4-mimetic miniprotein M48-U1, with a cyclohexylmethoxyphenyl insertion into the Phe 43 cavity showed best neutralization properties. M48-U1 bound YU2 gp120 with 25 pM affinity and neutralized all circulating HIV-1 isolates, except clade A/E viruses. The crystal structure of M48-U1 bound toYU2 gp120 was solved at 1.49 A resolution. This was compared to the 2.1 A resolution structure of a novel CD4-mimetic miniprotein, M48-U7 that inserts a flexible 5-phenoxypentan-1-ol moiety into the Phe 43 cavity. Structural analyses reveal residual ligand flexibility in the bound state. The M48-U1/U7-bound Phe 43 cavity resembled the cavity conformation in unliganded gp120 coree. These results provide an understanding of the structure-function relationship of Phe 43 cavity filling and define a mechanism for the broad and potent neutralization by the CD4-mimetic miniprotein M48-U1.
(C) 2013 Lippincott Williams & Wilkins, Inc.