Role of a conserved tripeptide in the endodomain of Sindbis virus glycoprotein E2 in virus assembly and function

Interactions of the cytoplasmic domain of Sindbis virus E2 with nucleocapsid cores promote alphavirus budding

Alphavirus budding from the plasma membrane occurs through the specific interaction of the nucleocapsid core with the cytoplasmic domain of the E2 glycoprotein (cdE2). Structural studies of the Sindbis virus capsid protein (CP) have suggested that these critical interactions are mediated by the binding of cdE2 into a hydrophobic pocket in the CP. Several molecular genetic studies have implicated amino acids Y400 and L402 in cdE2 as important for the budding of alphaviruses. In this study, we characterized the role of cdE2 residues in structural polyprotein processing, glycoprotein transport, and capsid interactions. Along with hydrophobic residues, charged residues in the N terminus of cdE2 were critical for the effective interaction of cores with cdE2, a process required for virus budding. Mutations in the C-terminal signal sequence region of cdE2 affected E2 protein transport to the plasma membrane, while nonbudding mutants that were defective in cdE2-CP interaction accumulated E2 on the plasma membrane. The interaction of cdE2 with cytoplasmic cores purified from infected cells and in vitro-assembled core-like particles suggests that cdE2 interacts with assembled cores to mediate budding. We hypothesize that these cdE2 interactions induce a change in the organization of the nucleocapsid core upon binding leading to particle budding and priming of the nucleocapsid cores for disassembly that is required for virus infection.

The structure of barmah forest virus as revealed by cryo-electron microscopy at a 6-angstrom resolution has detailed transmembrane protein architecture and interactions

Barmah Forest virus (BFV) is a mosquito-borne alphavirus that infects humans. A 6-Å-resolution cryo-electron microscopy three-dimensional structure of BFV exhibits a typical alphavirus organization, with RNA-containing nucleocapsid surrounded by a bilipid membrane anchored with the surface proteins E1 and E2. The map allows details of the transmembrane regions of E1 and E2 to be seen. The C-terminal end of the E2 transmembrane helix binds to the capsid protein. Following the E2 transmembrane helix, a short α-helical endodomain lies on the inner surface of the lipid envelope. The E2 endodomain interacts with E1 transmembrane helix from a neighboring E1-E2 trimeric spike, thereby acting as a spacer and a linker between spikes. In agreement with previous mutagenesis studies, the endodomain plays an important role in recruiting other E1-E2 spikes to the budding site during virus assembly. The E2 endodomain may thus serve as a target for antiviral drug design.

Generating enveloped virus-like particles with in vitro assembled cores

Alphaviruses are comprised of a nucleocapsid core surrounded by a lipid membrane containing glycoprotein spikes. Previous work demonstrated that in vitro assembled core-like particles are similar in structure to the nucleocapsid core in the native virus. Here we demonstrate that in vitro assembled core-like particles can be inserted into viral glycoprotein-expressing cells to generate enveloped virus-like particles. These virus-like particles bud from cells like native virus, are similar in size to the native virus, and can enter cells to release the contents of the core-like particle into the cytoplasm of the cell. Virus-like particles can be used to infect cells with biological and non-biological cargoes. The generation of enveloped virus-like particles containing an in vitro core and in vivo synthesized glycoproteins has applications for gene and drug delivery, medical imaging, and also basic mechanistic studies of virus assembly.

Location and role of free cysteinyl residues in the Sindbis virus E1 and E2 glycoproteins

Sindbis virus is a single-stranded positive-sense RNA virus. It is composed of 240 copies of three structural proteins: E1, E2, and capsid. These proteins form a mature virus particle composed of two nested T=4 icosahedral shells. A complex network of disulfide bonds in the E1 and E2 glycoproteins is developed through a series of structural intermediates as virus maturation occurs (M. Mulvey and D. T. Brown, J. Virol. 68:805-812, 1994; M. Carleton et al., J. Virol. 71:1558-1566, 1997). To better understand the nature of this disulfide network, E1 and E2 cysteinyl residues were labeled with iodoacetamide in the native virus particle and analyzed by liquid chromatography-tandem mass spectrometry. This analysis identified cysteinyl residues of E1 and E2, which were found to be label accessible in the native virus particle, as well as those that were either label inaccessible or blocked by their involvement in disulfide bonds. Native virus particles alkylated with iodoacetamide demonstrated a 4-log decrease in viral infectivity. This suggests that the modification of free cysteinyl residues results in the loss of infectivity by destabilizing the virus particle or that a rearrangement of disulfide bonds, which is required for infectivity, is blocked by the modification. Although modification of these residues prevented infectivity, it did not alter the ability of virus to fuse cells after exposure to acidic pH; thus, modification of free cysteinyl residues biochemically separated the process of infection from the process of membrane fusion.

Mutations in the endodomain of Sindbis virus glycoprotein E2 define sequences critical for virus assembly

Envelopment of Sindbis virus at the plasma membrane is a multistep process in which an initial step is the association of the E2 protein via a cytoplasmic endodomain with the preassembled nucleocapsid. Sindbis virus is vectored in nature by blood-sucking insects and grows efficiently in a number of avian and mammalian vertebrate hosts. The assembly of Sindbis virus, therefore, must occur in two very different host cell environments. Mammalian cells contain cholesterol which insect membranes lack. This difference in membrane composition may be critical in determining what requirements are placed on the E2 tail for virus assembly. To examine the interaction between the E2 tail and the nucleocapsid in Sindbis virus, we have produced substitutions and deletions in a region of the E2 tail (E2 amino acids 408 to 415) that is initially integrated into the endoplasmic reticulum. This sequence was identified as being critical for nucleocapsid binding in an in vitro peptide protection assay. The effects of these mutations on virus assembly and function were determined in both vertebrate and invertebrate cells. Amino acid substitutions (at positions E2: 408, 410, 411, and 413) reduced infectious virus production in a position-dependent fashion but were not efficient in disrupting assembly in mammalian cells. Deletions in the E2 endodomain (delta406-407, delta409-411, and delta414-417) resulted in the failure to assemble virions in mammalian cells. Electron microscopy of BHK cells transfected with these mutants revealed assembly of nucleocapsids that failed to attach to membranes. However, introduction of these deletion mutants into insect cells resulted in the assembly of virus-like particles but no assayable infectivity. These data help define protein interactions critical for virus assembly and suggest a fundamental difference between Sindbis virus assembly in mammalian and insect cells.