Structural mechanism of trimeric HIV-1 envelope glycoprotein activation.
HIV-1 infection begins with the binding of trimeric viral envelope glycoproteins (Env) to CD4 and a co-receptor on target T-cells. Understanding how these ligands influence the structure of Env is of fundamental interest for HIV vaccine development. Using cryo-electron microscopy, we describe the contrasting structural outcomes of trimeric Env binding to soluble CD4, to the broadly neutralizing, CD4-binding site antibodies VRC01, VRC03 and b12, or to the monoclonal antibody 17b, a co-receptor mimic. Binding of trimeric HIV-1 BaL Env to either soluble CD4 or 17b alone, is sufficient to trigger formation of the open quaternary conformation of Env. In contrast, VRC01 locks Env in the closed state, while b12 binding requires a partial opening in the quaternary structure of trimeric Env. Our results show that, despite general similarities in regions of the HIV-1 gp120 polypeptide that contact CD4, VRC01, VRC03 and b12, there are important differences in quaternary structures of the complexes these ligands form on native trimeric Env, and potentially explain differences in the neutralizing breadth and potency of antibodies with similar specificities. From cryo-electron microscopic analysis at ∼9 Å resolution of a cleaved, soluble version of trimeric Env, we show that a structural signature of the open Env conformation is a three-helix motif composed of α-helical segments derived from highly conserved, non-glycosylated N-terminal regions of the gp41 trimer. The three N-terminal gp41 helices in this novel, activated Env conformation are held apart by their interactions with the rest of Env, and are less compactly packed than in the post-fusion, six-helix bundle state. These findings suggest a new structural template for designing immunogens that can elicit antibodies targeting HIV at a vulnerable, pre-entry stage.
Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, Maryland, United States of America.
Erin E H
EE
Tran
Mario J
MJ
Borgnia
Oleg
O
Kuybeda
David M
DM
Schauder
Alberto
A
Bartesaghi
Gabriel A
GA
Frank
Guillermo
G
Sapiro
Jacqueline L S
JL
Milne
Sriram
S
Subramaniam
D000911:Antibodies, Monoclonal / Q000378:metabolism; D001665:Binding Sites; D001666:Binding Sites, Antibody; D015704:CD4 Antigens / Q000378:metabolism; D020285:Cryoelectron Microscopy; D015699:HIV Envelope Protein gp120 / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism*; D015497:HIV-1 / Q000378:metabolism* / Q000648:ultrastructure; D008024:Ligands; D008958:Models, Molecular; D018716:Molecular Mimicry; D011485:Protein Binding; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D015728:Receptors, HIV / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism
D000911:Antibodies, Monoclonal; D015704:CD4 Antigens; D015699:HIV Envelope Protein gp120; D008024:Ligands; D015728:Receptors, HIV; D011994:Recombinant Proteins; C076613:recombinant soluble CD4