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J. Virol., Apr 1997, 3188-3196, Vol 71, No. 4
JL Liu, LF Lee, Y Ye, Z Qian and HJ Kung
Marek's disease virus (MDV) is one of the most oncogenic herpesviruses and
induces T lymphomas in chickens within weeks after infection. Only a
limited number of viral transcripts are detected in MDV tumor samples and
cell lines. One of the major transcripts encodes MEQ, a 339-amino- acid
bZIP protein which is homologous to the Jun/Fos family of transcription
factors. The C-terminal half of MEQ contains proline-rich repeats and, when
fused to the DNA-binding domain of a yeast transcription factor, Gal4
(residues 1 to 147), exhibits transactivation function. MEQ can dimerize
with itself and with c-Jun. The MEQ-c-Jun heterodimers bind to an AP-1-like
enhancer within the MEQ promoter region with greater affinity than do
homodimers of either protein, and they transactivate MEQ expression. Here
we show that MEQ is expressed in the nucleus but, interestingly, with a
predominant fraction in the nucleoli and coiled bodies. This makes MEQ the
first bZIP protein to be identified in the nucleoli. MEQ contains two
stretches of basic residues, designated basic region 1 (BR1) and basic
region 2 (BR2). Using a series of deletion mutants, we have mapped the
primary nuclear localization signal (NLS) and the sole nucleolar
localization signal (NoLS) to the BR2 region. BR1 was shown to provide an
auxiliary signal in nuclear translocation. To demonstrate that BR2 is an
authentic NoLS, BR2 was fused to cytoplasmic v-Raf (delta gag) kinase. The
BR2-Raf fusion protein was observed to migrate into the nucleoplasm and the
nucleolus. The BR2 region can be further divided into two long
arginine-lysine stretches, BR2N and BR2C, which are separated by the five
amino acids Asn-Arg-Asp-Ala-Ala (NRDAA). We provide evidence that the
requirement for nuclear translocation is less stringent than that for
nucleolar translocation, as either BR2N or BR2C alone is sufficient to
translocate the cytoplasmic v-Raf (delta gag) into the nucleus, but only in
combination can they translocate v-Raf (delta gag) into the nucleolus. Our
studies demonstrate that MEQ is both a nuclear and nucleolar protein,
adding MEQ to the growing list of transactivators which localize to the
nucleolus.
Copyright © 1997, American Society for Microbiology
Nucleolar and nuclear localization properties of a herpesvirus bZIP oncoprotein, MEQ
Department of Molecular Biology and Microbiology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4960, USA.
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