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Journal of Virology, November 1999, p. 9532-9543, Vol. 73, No. 11
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Mutant Cells Selected during Persistent Reovirus
Infection Do Not Express Mature Cathepsin L and Do Not Support
Reovirus Disassembly
Geoffrey S.
Baer,1,2
Daniel H.
Ebert,1,2
Chia J.
Chung,3
Ann H.
Erickson,3 and
Terence
S.
Dermody1,2,4,*
Departments of Microbiology and
Immunology1 and of
Pediatrics4 and Elizabeth B. Lamb Center for Pediatric Research,2 Vanderbilt
University School of Medicine, Nashville, Tennessee 37232, and
Department of Biochemistry and Biophysics, University of
North Carolina at Chapel Hill, Chapel Hill, North Carolina
275993
Received 6 April 1999/Accepted 9 August 1999
Persistent reovirus infections of murine L929 cells select cellular
mutations that inhibit viral disassembly within the endocytic pathway.
Mutant cells support reovirus growth when infection is initiated with
infectious subvirion particles (ISVPs), which are intermediates in
reovirus disassembly formed following proteolysis of viral outer-capsid
proteins. However, mutant cells do not support growth of virions,
indicating that these cells have a defect in virion-to-ISVP processing.
To better understand mechanisms by which viruses use the endocytic
pathway to enter cells, we defined steps in reovirus replication
blocked in mutant cells selected during persistent infection.
Subcellular localization of reovirus after adsorption to parental and
mutant cells was assessed using confocal microscopy and virions
conjugated to a fluorescent probe. Parental and mutant cells did not
differ in the capacity to internalize virions or distribute them to
perinuclear compartments. Using pH-sensitive probes, the intravesicular
pH was determined and found to be equivalent in parental and mutant
cells. In both cell types, virions localized to acidified intracellular
organelles. The capacity of parental and mutant cells to support
proteolysis of reovirus virions was assessed by monitoring the
appearance of disassembly intermediates following adsorption of
radiolabeled viral particles. Within 2 h after adsorption to
parental cells, proteolysis of viral outer-capsid proteins was
observed, consistent with formation of ISVPs. However, in mutant cells,
no proteolysis of viral proteins was detected up to 8 h
postadsorption. Since treatment of cells with E64, an inhibitor of
cysteine-containing proteases, blocks reovirus disassembly, we used
immunoblot analysis to assess the expression of cathepsin L, a
lysosomal cysteine protease. In contrast to parental cells, mutant
cells did not express the mature, proteolytically active form of the
enzyme. The defect in cathepsin L maturation was not associated with
mutations in procathepsin L mRNA, was not complemented by procathepsin
L overexpression, and did not affect the maturation of cathepsin B,
another lysosomal cysteine protease. These findings indicate that
persistent reovirus infections select cellular mutations that affect
the maturation of cathepsin L and suggest that alterations in the
expression of lysosomal proteases can modulate viral cytopathicity.
*
Corresponding author. Mailing address: Lamb Center for
Pediatric Research, D7235 MCN, Vanderbilt University School of
Medicine, Nashville, TN 37232. Phone: (615) 343-9943. Fax: (615)
343-9723. E-mail:
terry.dermody{at}mcmail.vanderbilt.edu.
Journal of Virology, November 1999, p. 9532-9543, Vol. 73, No. 11
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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