Specific Interaction of a Novel Foamy Virus Env Leader Protein with the N-Terminal Gag Domain

Abstract

Cryoelectron micrographs of purified human foamy virus (HFV) and feline foamy virus (FFV) particles revealed distinct radial arrangements of Gag proteins. The capsids were surrounded by an internal Gag layer that in turn was surrounded by, and separated from, the viral membrane. The width of this layer was about 8 nm for HFV and 3.8 nm for FFV. This difference in width is assumed to reflect the different sizes of the HFV and FFV MA domains: the HFV MA domain is about 130 residues longer than that of FFV. The distances between the MA layer and the edge of the capsid were identical in different particle classes. In contrast, only particles with a distended envelope displayed an invariant, close spacing between the MA layer and the Env membrane which was absent in the majority of particles. This indicates a specific interaction between MA and Env at an unknown step of morphogenesis. This observation was supported by surface plasmon resonance studies. The purified N-terminal domain of FFV Gag specifically interacted with synthetic peptides and a defined protein domain derived from the N-terminal Env leader protein. The specificity of this interaction was demonstrated by using peptides varying in the conserved Trp residues that are known to be required for HFV budding. The interaction with Gag required residues within the novel virion-associated FFV Env leader protein of about 16.5 kDa. The genomes of spumaretroviruses (or foamy viruses [FV])include the classical gag, pro-pol, and env genes that are the hallmark of the retrovirus family (9). Despite the familial relationship implied by their genome organization, FV differ from retroviruses such as oncoviruses or lentiviruses in basic aspects of replication and gene expression (30-32, 39, 53). The differences between FV and the more widely studied retroviruses, such as human immunodeficiency virus (HIV) or murine leukemia virus, provide an opportunity to identify the fundamental mechanisms of processes such as particle assembly and maturation.The shared gene order of the FV genomes and those of other retroviruses allows the identification of common structural proteins. Unfortunately, this relationship does not extend to the structure and function of FV Gag proteins, since no obvious homologies are detectable between FV Gag and MA, CA, and NC domains of other retroviruses for which structures have been determined to high resolution (10, 43). Furthermore, the proteolytic processing of Gag that produces the familiar mature proteins in other retroviruses is unusual, incomplete, and/or delayed in FV (12,23,29,37).The available data indicate that the morphology and morphogenesis of FV are distinct from those of other retroviruses. FV Gag proteins preassemble to form in the cytoplasm spherical capsids that bud through cellular membranes (55). This process contrasts with what occurs with lentiviruses and C-type retroviruses that assemble their capsids at the site of budding but is similar to what occurs with B-and D-type retroviruses (40). Budding of human foamy virus (HFV) i...

Do lipid rafts mediate virus assembly and pseudotyping?

“…1(A). The FFV particle (black annotation) displays the doublelayered angular core structure (hexagon, small arrowheads), clear membrane leaflets (arrows) and prominent rod-shaped spikes (~130 Å long, large arrowhead) that characterize FFV in cryo-electron microscopy (Wilk et al, 2000(Wilk et al, , 2001a. The FLV particle is annotated in white.…”

Construction and characterization of efficient, stable and safe replication-deficient foamy virus vectors

“…These results show that type-specific interactions between N-terminal Env leader protein sequences and Gag are required as recently proposed although other factors may be also required. 29,30 We also tested whether FFV particles can be pseudotyped with the vesicular stomatitis virus glycoprotein VSV-G. 31 In repeated experiments, no marker gene transfer using VSV-G-pseudotyped FFV vectors was detectable confirming previous studies using simian/ primate FVs. 24 …”

“…Even the Env protein from the human/primate FV did not allow efficient marker gene transduction, underlining the concept that highly specific Gag-Env interactions are required for proper particle assembly although other reasons cannot be formally excluded. 29,30 In addition, we did not find a significant effect of Bet on the vector titer, as the 293T cells used for vector production are APOBEC3-deficient and thus, the deaminase-counteracting function of Bet is not required. 22 Additional effects of Bet on FFV vector particle release were not observed in this study.…”

“…Particles were sedimented through 2 ml of 20% (wt/vol) sucrose in phosphate buffer saline (PBS) for 2 h at 28 000 r.p.m in a SW 41 rotor (Beckman, Munich, Germany) and resuspended in protein lysis buffer. 30 …”

Immunising with the transmembrane envelope proteins of different retroviruses including HIV-1

“…In all animals binding peptide of FV Env, designated Elp, is an important component of FFV and PFV particles. [68][69][70] The C-terminus of Elp is exposed on the surface of virus particles and may be thus very much suited to display vaccine epitopes to the vaccines immune system. 69 Importantly, Env of PFV has been described to bud from cells, 65 a feature that could be used to create vaccine virus-like particles (VLPs) or a particulate protein vaccine.…”