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Journal of Virology, July 2001, p. 5891-5898, Vol. 75, No. 13
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.13.5891-5898.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Immunoreceptor Tyrosine-Based Activation
Motif-Dependent Signaling by Kaposi's Sarcoma-Associated Herpesvirus
K1 Protein: Effects on Lytic Viral Replication
Michael
Lagunoff,
David M.
Lukac,
and
Don
Ganem*
Howard Hughes Medical Institute, Departments
of Microbiology and Immunology and Medicine, University of
California Medical Center, San Francisco, California 94143-0414
Received 5 February 2001/Accepted 3 April 2001
The Kaposi's sarcoma-associated herpesvirus (KSHV) K1 gene encodes
a polypeptide bearing an immunoreceptor tyrosine-based activation motif
(ITAM) that is constitutively active for ITAM-based signal
transduction. Although ectopic overexpression of K1 in cultured
fibroblasts can lead to growth transformation, in vivo this gene is
primarily expressed in lymphoid cells undergoing lytic infection. Here
we have examined function of K1 in the setting of lytic replication,
through the study of K1 mutants lacking functional ITAMs. Expression of
such mutants in BJAB cells cotransfected with wild-type K1 results in
dramatic inhibition of K1 signal transduction, as judged by impaired
activation of Syk kinase and phospholipase C-
2 as well as by
diminished expression of a luciferase reporter gene dependent upon
K1-induced calcium and Ras signaling. Thus, the mutants behave as
dominantly acting inhibitors of K1 function. To assess the role of K1
in lytic replication, we introduced these K1 mutants into BCBL-1 cells,
a B-cell lymphoma line latently infected with KSHV, and induced lytic
replication by ectopic expression of the KSHV ORF50 transactivator.
Expression of lytic cycle genes was diminished up to 80% in the
presence of a K1 dominant negative mutant. These inhibitory effects
could be overridden by tetradecanoyl phorbol acetate treatment,
indicating that inhibition was not due to irreversible cell injury and
suggesting that other signaling events could bypass the block. We
conclude that ITAM-dependent signaling by K1 is not absolutely required
for lytic reactivation but functions to modestly augment lytic
replication in B cells, the natural reservoir of KSHV.
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, School of Medicine, University of
California, San Francisco, San Francisco, CA 94143-0414. Phone: (415)
476-2826. Fax: (415) 476-0939. E-mail: ganem{at}cgl.ucsf.edu.

Present address: Department of Microbiology, University of
Washington, Seattle, WA
98195.

Present address: Department of Microbiology and Molecular
Genetics, UMDNJ/NJ Medical School, Newark, NJ
07103.
Journal of Virology, July 2001, p. 5891-5898, Vol. 75, No. 13
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.13.5891-5898.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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