Journal of Molecular Biology
Three-dimensional Structure of HIV-1 Virus-like Particles by Electron Cryotomography
Introduction
While high-resolution structures of many viruses have now been determined by X-ray crystallography, such models will not be obtainable for the type 1 human immunodeficiency virus (HIV-1) because virions are individually unique. Instead, molecular models for HIV-1 must for now be obtained by combining high-resolution structural studies of isolated viral components together with lower-resolution reconstructions of intact virions generated by electron microscopy (EM). Thus, our understanding of HIV-1 structure has advanced stepwise with the development and application of ever more sophisticated EM methods. After the virus was first identified in the early 1980s, its gross structure and organization were quickly characterized using the traditional EM techniques of thin sectioning and immuno-staining or negative staining.1, 2 Structural models of the mature virion have subsequently been refined using data from cryo-EM,3 STEM,4 and tomography.5, 6
We now know that the mature HIV-1 virion has a roughly spherical outer lipid bilayer 120–200 nm in diameter,3 which is studded with trimeric clusters of the transmembrane Env protein.6 Inside the bilayer, there is a series of structural shells that organize the viral RNA genome and its associated enzymes for uncoating and replication in a new host cell. These different shells are assembled during the process of viral maturation, when the immature Gag polyprotein is processed proteolytically to produce the smaller MA, CA, and NC proteins (as well as several smaller peptides). The outermost shell of the mature virus (the matrix) is associated with the inner face of the bilayer and is composed of ∼4000–5000 copies of the N-myristoylated MA protein. Inside the matrix is an unusual conical particle (the core), whose conical outer shell (the capsid) is composed of ∼1000–1500 copies of the viral CA protein. Within the capsid is a ribonucleoprotein (RNP) particle composed of two copies of the positive-sense RNA genome, thousands of copies of the RNA-binding NC protein, and ∼250 copies each of the reverse transcriptase (RT) and integrase (IN) enzymes.4
High-resolution structural models are now available for essentially all of the HIV-1 proteins or their constituent domains, including MA, CA, and NC.7 Moreover, molecular models for higher-order interactions within the viral matrix8, 9, 10 and capsid11 lattices have been proposed on the basis of structural analyses of recombinant MA and CA protein assemblies. In the case of the capsid, EM analyses of helical and two-dimensional crystals of HIV-1 CA have revealed that the protein assembles preferentially into p6-based lattices composed of CA hexamers.11, 12 It has been proposed that the conical viral capsid assembles on a similar hexagonal CA net following the structural principles of a fullerene cone.13, 14, 15, 16 In the fullerene cone model, the body of the conical capsid is composed of CA hexamers, and the two ends of the cone are allowed to close through the introduction of pentameric defects, with five pentons at the narrow end and seven pentons at the wide end. The fullerene cone model correctly predicts the observed quantization of the cone angles of synthetic capsid assemblies,15 and is supported strongly by a series of cryo-EM measurements made on authentic viral cores.3 However, all of these cone angle measurements were made with simple projection images, and this precluded a direct comparison of actual capsid cone angles to the quantized values predicted by the fullerene cone model, because the authentic capsids could adopt variable orientations with respect to the electron beam,3 and because the synthetic capsids were flattened and possibly distorted in other ways when dried onto carbon films.15
The recent development of electron cryotomography provides a new and more powerful way to investigate HIV-1 structure. This method offers the advantages inherent in cryo-EM, in which a sample in solution is spread into a thin film across an EM grid and plunged into liquid ethane.17 This procedure preserves the sample in a near-native, “frozen-hydrated” state, and allows the actual structure of the biological macromolecules to be imaged through their inherent contrast against an aqueous background. In tomography, a series of images is recorded of a specimen while it is tilted incrementally about some axis. Three-dimensional reconstructions of the specimen (tomograms) are then calculated by merging the images by back-projection in real space, or by Fourier interpolation in reciprocal space. While the potential advantages of electron cryotomography have been discussed for decades, only in the past few years has it become practically feasible and effective through the advent of high-quality CCD cameras, motorized and computer-controlled stages, programmable EMs, and development of automatic protocols to control specimen tracking, focusing, and exposures.18, 19, 20
Two tomographic analyses of HIV particles have been reported, yielding information on the structure of the virion,5 and on the shape and distribution of envelope glycoproteins on the virion surface.6 Both of these studies, however, utilized negative staining procedures that limited the accurate visualization of internal core structures. Here, we report the first application of electron cryotomography to determine the three-dimensional structure of several tens of individual, unfixed, unstained, whole HIV-1 virus-like particles (VLPs).
Section snippets
Results
Non-infectious HIV-1 VLPs were produced from a proviral expression construct with debilitating mutations in the RT and RNase H enzymes and a frameshift mutation that prevented Env protein expression. VLPs were produced in cultured human 293T cells, recovered from the supernatant, and purified by sucrose-density centrifugation. As expected, the HIV-1 CA, MA, and NC proteins were the major protein components in the sample as analyzed by SDS-PAGE (data not shown).
HIV-1 VLPs were plunge-frozen onto
Discussion
The architecture of the layers of HIV-1 and their morphogenesis are intrinsically interesting from a structural point of view, and may lead to novel therapeutics that block assembly or maturation. Indeed, several small molecules have been reported to block structural transitions required for the assembly or maturation of several viruses, including HIV-1.24, 25, 26 Unfortunately, the architecture of HIV-1 has been difficult to study, because each virus is unique, and therefore traditional X-ray
Preparation of VLP
HIV-1 VLPs were made non-infectious by inactivation of the essential Env, RT, and RNase H proteins. An HIV-1NL4-3 R9 proviral vector38 with a frameshift mutation in the env gene that blocked Env protein production was obtained as a gift from Dr Christopher Aiken (Vanderbilt University). Although Env deletion alone blocks viral infectivity, two additional mutations were added that independently block viral replication. First, the viral RT was inactivated with a D185A (RTD185A) mutation. D185 is
Acknowledgements
This work was supported by NIH grant PO1 GM66521 to W.I.S. and G.J.J., as well as gifts from the Ralph M. Parsons Foundation, the Agouron Institute, and the Gordon and Betty Moore Foundation to the California Institute of Technology. We are grateful to Uta von Schwedler and Kirsten Stray for assistance with VLP preparations, and we thank Simon Wain–Hobsen for suggesting that we estimate the concentrations of enzymes in the viral core.
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