Molecular architecture of native HIV-1 gp120 trimers

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Abstract
This paper investigates the structure of the HIV the gp120 coat protein of HIV by cryo-electron tomography and molecular modelling. Comparison of gp120 structures in an unbound state, bound to a neutralizing antibody and bound to CD4 cell surface protein provides insight into the conformational changes that occur during antibody neutralization and attachment to target cells. This paper investigates the structure of the HIV glycoprotein gp120 by cryo-electron tomography and molecular modelling. gp120 is analysed in an unliganded state, complexed with a neutralizing antibody and in a CD4 liganded state. The analysis provides insight into the conformational changes that occur with ligand binding. The envelope glycoproteins (Env) of human and simian immunodeficiency viruses (HIV and SIV, respectively) mediate virus binding to the cell surface receptor CD4 on target cells to initiate infection1. Env is a heterodimer of a transmembrane glycoprotein (gp41) and a surface glycoprotein (gp120), and forms trimers on the surface of the viral membrane. Using cryo-electron tomography combined with three-dimensional image classification and averaging, we report the three-dimensional structures of trimeric Env displayed on native HIV-1 in the unliganded state, in complex with the broadly neutralizing antibody b12 and in a ternary complex with CD4 and the 17b antibody. By fitting the known crystal structures2,3 of the monomeric gp120 core in the b12- and CD4/17b-bound conformations into the density maps derived by electron tomography, we derive molecular models for the native HIV-1 gp120 trimer in unliganded and CD4-bound states. We demonstrate that CD4 binding results in a major reorganization of the Env trimer, causing an outward rotation and displacement of each gp120 monomer. This appears to be coupled with a rearrangement of the gp41 region along the central axis of the trimer, leading to closer contact between the viral and target cell membranes. Our findings elucidate the structure and conformational changes of trimeric HIV-1 gp120 relevant to antibody neutralization and attachment to target cells.