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Journal of Virology, April 1999, p. 3246-3257, Vol. 73, No. 4
Department of Microbiology and Molecular
Genetics, University of California, Irvine, California 92697-4025
Received 31 July 1998/Accepted 14 January 1999
The herpes simplex virus type 1 (HSV-1) regulatory protein ICP27 is
a 63-kDa phosphoprotein required for viral replication. ICP27 has been
shown to contain both stable phosphate groups and phosphate groups that
cycle on and off during infection (K. W. Wilcox, A. Kohn, E. Sklyanskaya, and B. Roizman, J. Virol. 33:167-182, 1980). Despite
extensive genetic analysis of the ICP27 gene, there is no information
available about the sites of the ICP27 molecule that are phosphorylated
during viral infection. In this study, we mapped several of the
phosphorylation sites of ICP27 following in vivo radiolabeling.
Phosphoamino acid analysis showed that serine is the only amino acid
that is phosphorylated during infection. Two-dimensional phosphopeptide
mapping showed a complex tryptic phosphopeptide pattern with at least
four major peptides and several minor peptides. In addition, ICP27
purified from transfected cells yielded a similar phosphopeptide
pattern, suggesting that cellular kinases phosphorylate ICP27 during
viral infection. In vitro labeling showed that protein kinase A (PKA),
PKC, and casein kinase II (CKII) were able to differentially
phosphorylate ICP27, resulting in distinct phosphopeptide patterns. The
major phosphorylation sites of ICP27 appeared to cluster in the
N-terminal portion of the protein, such that a frameshift mutant that
encodes amino acids 1 to 163 yielded a phosphopeptide pattern very
similar to that seen with the wild-type protein. Further, using small
deletion and point mutations in kinase consensus sites, we have
elucidated individual serine residues that are phosphorylated in vivo.
Specifically, the serine at residue 114 was highly phosphorylated by
PKA and the serine residues at positions 16 and 18 serve as targets for CKII phosphorylation in vivo. These kinase consensus site mutants were still capable of complementing the growth of an
ICP27-null mutant virus. Interestingly, phosphorylation of the serine
at residue 114, which lies within the major nuclear localization signal, appeared to modulate the efficiency of nuclear import of ICP27.
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Analysis of the Phosphorylation Sites of Herpes
Simplex Virus Type 1 Regulatory Protein ICP27
*
Corresponding author. Mailing address: Department of
Microbiology and Molecular Genetics, College of Medicine, B240 Medical Sciences I, University of California, Irvine, CA 92697-4025. Phone: (949) 824-7570. Fax: (949) 824-8598. E-mail:
RMSANDRI{at}UCI.EDU.
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