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J. Virol., Oct 1997, 7227-7239, Vol 71, No. 10
DM Lukac, NY Harel, N Tanese and JC Alwine
The human cytomegalovirus (HCMV) major immediate-early (IE) proteins IEP86
(IE2(579aa)) and IEP72 (IE1(491aa)) can transcriptionally activate a
variety of simple promoters containing a TATA element and one upstream
transcription factor binding site. In our previous studies, transcriptional
activation was shown to correlate with IEP86 binding to both the TATA-box
binding protein (TBP) and the transcription factor bound upstream. IEP72
often synergistically affects the activation by IEP86, although it has not
previously been shown to directly interact in vitro with IEP86, TBP, or
transcription factors (e.g., Sp1 and Tef-1) bound by IEP86. We report
biochemical and genetic evidence suggesting that the major IE proteins may
perform a function similar to that of the TBP-associated factors (TAFs)
which make up TFIID. Consistent with this model, we found that the major IE
proteins interact with a number of TAFs. In vitro, IEP86 bound with
drosophila TAF(II)110 (dTAF(II)110) and human TAF(II)130 (hTAF(II)130),
while IEP72 bound dTAF(II)40, dTAF(II)110, and hTAF(II)130. Regions on
major IE proteins which mediate binding have been defined. In addition, our
data indicate that both IEP72 and IEP86 can bind simultaneously to
hTAF(II)130, suggesting that this TAF may provide bridging interactions
between the two proteins for transcriptional activation and synergy. In
agreement, a transcriptional activation mutant of IEP72 is unable to
participate in bridging. Confirmation that these in vitro interactions were
relevant was provided by data showing that both IEP72 and IEP86 copurify
with TFIID and coimmunoprecipitate with purified TFIID derived from
infected cell nuclei. To further support a TAF-like function of the IE
proteins, we have found that the IE proteins expressed from the intact
major IE gene, and to a lesser extent IEP86 alone, can rescue the
temperature-sensitive (ts) transcriptional defect in TAF(II)250 in the
BHK-21 cell line ts13. Analyses of mutations in the major IE region show
that IEP86 is essential for rescue and that IEP72 augments its effect, and
that mutations which affect TAF interactions are debilitated in rescue. Our
data, showing that the IE proteins can bind with TFIID and rescue a ts
transcriptional defect in TAF(II)250, support the model that the IE
proteins perform a TAF-like function as components of TFIID.
Copyright © 1997, American Society for Microbiology
TAF-like functions of human cytomegalovirus immediate-early proteins
Department of Microbiology and Graduate Group of Molecular Biology, School of Medicine, University of Pennsylvania, Philadelphia 19104- 6142, USA.
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