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Journal of Virology, October 1999, p. 8917-8917, Vol. 73, No. 10
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
LETTERS TO THE EDITOR
Endogenous Virus of BHK-21 Cells Complicates Electron
Microscopy Studies of Foamy Virus Maturation
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LETTER |
Baldwin and Linial previously reported that in the absence of the
envelope glycoprotein (Env), foamy virus (FV) particles were not
released from BHK-21-derived FAB cells as determined by
immunoprecipitation of FV proteins from culture media (2). These results were similar to those of Fischer et al. (5)
who utilized 293T cells. Budding of Env-deficient viral particles from
the plasma membrane (PM) was not seen by electron microscopy (EM) in
either report. However, these two reports differed regarding EM
evidence for intracellular FV budding in the absence of Env. Baldwin
and Linial found a few examples of intracytoplasmic budding of
Env-deficient FV particles in BHK-21-derived FAB cells, whereas Fischer
et al. reported no such budding in 293T cells. Is there an explanation
for this apparent discrepancy?
FVs are known to bud both intracellularly and at the PM (9,
12), in contrast to other exogenous retroviruses which regularly mature solely at the PM. FVs are also unique among retroviruses in that
they possess an endoplasmic reticulum (ER) retrieval signal in Env
(6, 8, 16). When this ER sorting motif was disrupted by
mutagenesis, increased budding at the PM was observed by EM relative to
wild-type FV (7). This result indicated a role for Env in
partitioning the site of FV budding to intracytoplasmic membranes.
The published EMs of Env-deficient FV mutants in
BHK-21-derived FAB cells (see Fig. 4D and E of reference
2) revealed viral particles within, or in the
process of budding into, intracellular compartments. These EMs were
presented to support the conclusion that Env-deficient FV can bud
intracellularly. However, on further examination, it was
recognized that the morphology of the particles in these images
differed from those of the FVs in all other panels (see Fig. 4A,
B, C, and F of reference 2). The particles in panels D and E possessed an electron-dense, mature-appearing viral core, as opposed to a characteristic electron-lucent,
immature-appearing viral core typical of FVs (1, 9).
The literature contains a number of reports that endogenous viruses are
present in BHK-21 cell lines (3, 4, 11, 13). These particles
were termed "intracisternal R-type particles" (IRPs)
(15) because of their unique morphology: spoke-like
structures radiating (thus the R type) from the electron-dense
mature-appearing viral core to the periphery of the particles. Review
of the published EMs of these IRPs (3, 4, 10) revealed that
the morphology of the intracellular Env-deficient viral particles in
Fig. 4D and E (2) closely resembled that of the endogenous
virus of BHK-21 cells. They are similar in size; both have an
electron-dense core, from which lines radiate to the outer border of
the particles; and they occur in the lumen of the ER or within the
nuclear envelope and sometimes appear to be budding into these spaces.
On recent reexamination of the EMs of our control cells, i.e.,
untransfected BHK-21-derived FAB cells, we recognized the presence of
infrequent virus-like particles budding intracellularly. These mature-appearing, spikeless particles were morphologically
indistinguishable from both the endogenous IRPs in the literature and
the particles in panels D and E of Fig. 4 of reference
;22;2. We believe that the particles in panels D and E
of Fig. 4 (2) represent the endogenous virus of BHK-21
cells. It is therefore appropriate to conclude, as did Fischer et al.
and Pietschmann et al. (5, 14), that FV budding, both
intracellularly and at the PM, requires the viral envelope glycoprotein.
| |
FOOTNOTES |
*
Phone: (205) 975-8982
Fax: (205) 975-6027
E-mail: mulligan{at}uab.edu
| |
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| | | | |
George Wang
Mark J. Mulligan*
Departments of Medicine and Microbiology
University of Alabama at Birmingham
845 19th St. South, BBRB 220
Birmingham, Alabama 35294-2170
|
| | | | |
David N. Baldwin
Maxine L. Linial
Division of Basic Sciences
Fred Hutchinson Cancer Research Center
Department of Microbiology
University of Washington
Seattle, Washington 98195
|
Journal of Virology, October 1999, p. 8917-8917, Vol. 73, No. 10
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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