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J. Virol., 06 1996, 3706-3715, Vol 70, No. 6
M Sakalian, SD Parker, RA Weldon Jr and E Hunter
The assembly of retroviral particles is mediated by the product of the gag
gene; no other retroviral gene products are necessary for this process.
While most retroviruses assemble their capsids at the plasma membrane,
viruses of the type D class preassemble immature capsids within the
cytoplasm of infected cells. This has allowed us to determine whether
immature capsids of the prototypical type D retrovirus, Mason-Pfizer monkey
virus (M-PMV), can assemble in a cell- free protein synthesis system. We
report here that assembly of M-PMV Gag precursor proteins can occur in this
in vitro system. Synthesized particles sediment in isopycnic gradients to
the appropriate density and in thin-section electron micrographs have a
size and appearance consistent with those of immature retrovirus capsids.
The in vitro system described in this report appears to faithfully mimic
the process of assembly which occurs in the host cell cytoplasm, since
M-PMV gag mutants defective in in vivo assembly also fail to assemble in
vitro. Likewise, the Gag precursor proteins of retroviruses that undergo
type C morphogenesis, Rous sarcoma virus and human immunodeficiency virus,
which do not preassemble capsids in vivo, fail to assemble particles in
this system. Additionally, we demonstrate, with the use of anti-Gag
antibodies, that this cell-free system can be utilized for analysis in
vitro of potential inhibitors of retrovirus assembly.
Copyright © 1996, American Society for Microbiology
Synthesis and assembly of retrovirus Gag precursors into immature capsids in vitro
Department of Microbiology, University of Alabama at Birmingham, 35294- 2170, USA.
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