Abstract
The envelope glycoproteins of primate lentiviruses, including human and simian immunodeficiency viruses (HIV and SIV), are heterodimers of a transmembrane glycoprotein (usually gp41), and a surface glycoprotein (gp120), which binds CD4 on target cells to initiate viral entry. We have used electron tomography to determine the three-dimensional architectures of purified SIV virions in isolation and in contact with CD4+ target cells. The trimeric viral envelope glycoprotein surface spikes are heterogeneous in appearance and typically ∼120 Å long and ∼120 Å wide at the distal end. Docking of SIV or HIV-1 on the T cell surface occurs via a neck-shaped contact region that is ∼400 Å wide and consistently consists of a closely spaced cluster of five to seven rod-shaped features, each ∼100 Å long and ∼100 Å wide. This distinctive structure is not observed when viruses are incubated with T lymphocytes in the presence of anti-CD4 antibodies, the CCR5 antagonist TAK779, or the peptide entry inhibitor SIVmac251 C34. For virions bound to cells, few trimers were observed away from this cluster at the virion–cell interface, even in cases where virus preparations showing as many as 70 envelope glycoprotein trimers per virus particle were used. This contact zone, which we term the “entry claw”, provides a spatial context to understand the molecular mechanisms of viral entry. Determination of the molecular composition and structure of the entry claw may facilitate the identification of improved drugs for the inhibition of HIV-1 entry.
Highlights
Entry of HIV-1 into target cells involves the interaction of the surface glycoprotein gp120 with the cell surface receptor CD4 [1], a binding-induced structural change [2] in gp120 that creates the binding site for a cellular seven-transmembrane-helix co-receptor protein [3], followed by conformational changes [4] in the transmembrane glycoprotein gp41 that allow formation of the ‘‘pre-hairpin’’ conformation [5,6]
We have used electron tomographic approaches to analyze the 3-D architectures of simian immunodeficiency virus (SIV) and HIV-1 virions incubated with CD4þ T lymphocyte target cells to identify structural features of cell-bound viruses trapped at a stage prior to entry into target cells
To validate the approach used for structural analysis of infected cells using electron tomography of stained, plasticembedded specimens, we first carried out analysis of unstained purified viruses using cryo-electron tomography, and compared the resulting 3-D structures with those obtained from analysis of free virions in stained, plasticembedded specimens
Summary
Entry of HIV-1 into target cells involves the interaction of the surface glycoprotein gp120 (designated SU) with the cell surface receptor CD4 [1], a binding-induced structural change [2] in gp120 that creates the binding site for a cellular seven-transmembrane-helix co-receptor protein [3], followed by conformational changes [4] in the transmembrane glycoprotein gp (designated TM) that allow formation of the ‘‘pre-hairpin’’ conformation [5,6]. Electron tomography is a powerful approach for determining the three-dimensional (3-D) structures of large and heterogeneous sub-cellular assemblies at resolutions that are typically one to two orders of magnitude higher than those that can be currently achieved using light microscopy [9,10,11]. Because these assemblies are not generally amenable to analysis by crystallographic approaches, electron tomography provides tools to bridge the gap between cellular and molecular structure. We have used electron tomographic approaches to analyze the 3-D architectures of simian immunodeficiency virus (SIV) and HIV-1 virions incubated with CD4þ T lymphocyte target cells to identify structural features of cell-bound viruses trapped at a stage prior to entry into target cells
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
