Identification of a Conserved Residue of Foamy Virus Gag Required for Intracellular Capsid Assembly

Abstract

In contrast to all retroviruses but similar to the hepatitis B virus, foamy viruses (FV) require expression of the envelope protein for budding of intracellular capsids from the cell, suggesting a specific interaction between the Gag and Env proteins. Capsid assembly occurs in the cytoplasm of infected cells in a manner similar to that for the B-and D-type viruses; however, in contrast to these retroviruses, FV Gag lacks an N-terminal myristylation signal and capsids are not targeted to the plasma membrane (PM). We have found that mutation of an absolutely conserved arginine (Arg) residue at position 50 to alanine (R50A) of the simian foamy virus SFV cpz(hu) inhibits proper capsid assembly and abolishes viral budding even in the presence of the envelope (Env) glycoproteins. Particle assembly and extracellular release of virus can be restored to this mutant with the addition of an N-terminal Src myristylation signal (Myr-R50A), presumably by providing an alternate site for assembly to occur at the PM. In addition, the strict requirement of Env expression for capsid budding can be bypassed by addition of a PM-targeting signal to Gag. These results suggest that intracellular capsid assembly may be mediated by a signal akin to the cytoplasmic targeting and retention signal CTRS found in Mason-Pfizer monkey virus and that FV Gag has the inherent ability to assemble capsids at multiple sites like conventional retroviruses. The necessity of Env expression for particle egress is most probably due to the lack of a membrane-targeting signal within FV Gag to direct capsids to the PM for release and indicates that Gag-Env interactions are essential to drive particle budding.

Targeting of incoming retroviral Gag to the centrosome involves a direct interaction with the dynein light chain 8

“…Although surface Env glycoproteins are needed to enter target cells, interaction and subsequent self-multimerization of Gag molecules lead to capsid formation (Eastman and Linial, 2001;Tobaly-Tapiero et al, 2001). Several functional regions were identified along the scaffold Gag protein.…”

“…Several functional regions were identified along the scaffold Gag protein. There are three glycine(G)/arginine(R)-rich sequences, the so-called GR boxes, in the C-terminus that are implicated in viral nucleic acid binding and harbor a nuclear localization sequence (NLS) (Schliephake and Rethwilm, 1994;Yu et al, 1996), a 18 amino acid motif (amino acids 43-60) resembling the cytoplasmic targeting and retention signal (CTRS) of type D retroviruses that allows cytoplasmic targeting of Gag (Eastman and Linial, 2001) and, finally, a coiled-coil domain (amino acids that is necessary for Gag-Gag interaction (Tobaly-Tapiero et al, 2001). The last two motifs are required for capsid assembly.…”

Complete nucleotide sequence and evolutionary analysis of a Gorilla foamy virus

“…Two late domain PSAP motifs (at aa 214-217 and 280-283) and three YXXL motifs (at 109-112, 200-203 and 463-466) were identified. The latter were shown to be essential for particle assembly of PFV (Eastman & Linial, 2001). Gag encodes three glycine-arginine-rich boxes (502-507, 547-553 and 594-598), which contribute to the nuclear localization of Gag and are involved in the binding of nucleic acids.…”

Structural and Evolutionary Analysis of an Orangutan Foamy Virus

“…Gag contains three glycine-arginine-rich stretches (GR-I, GR-II, and GR-III; aa 462 to 492, 518 to 541, and 563 to 596, respectively) which are involved in the binding of nucleic acid and in the nuclear localization of the Gag precursor (29). A conserved arginine residue that acts in the intracellular capsid assembly is located at aa position 46 (9). A stretch of regularly spaced hydrophobic residues that may form a coiled-coil motif necessary for Gag-Gag interaction during viral capsid formation extends from aa 125 to 187 (36).…”