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J. Virol., Apr 1996, 2260-2268, Vol 70, No. 4
E Flory, A Hoffmeyer, U Smola, UR Rapp and JT Bruder
The serine/threonine protein kinase Raf-1 is a component of a conserved
intracellular signaling cascade that controls responses to various
extracellular stimuli. Transcription from several promoters, including the
oncogene-responsive element in the polyomavirus enhancer, the c-fos
promoter, as well as other AP-1- and Ets-dependent promoters, can be
induced by Raf-1 kinase. Previously, we have shown that activated Raf-1
kinase transactivates the human immunodeficiency virus type 1 (HIV-1) long
terminal repeat and have identified the NF-kappaB binding motif as a
Raf-1-responsive element (RafRE). We now report that Raf-1 kinase- induced
transactivation from the HIV RafRE involves the purine-rich- repeat-binding
protein (GABP), which is composed of two distinct subunits (alpha and
beta). GABP alpha is an Ets oncogene-related DNA- binding protein, and GABP
beta contains four ankyrin-like repeats that have been shown to be
essential in protein-protein interactions. In electrophoretic mobility
shift assays using nuclear extracts from human Jurkat T cells, a
protein-DNA complex which was supershifted with antiserum against GABP
alpha and GABP beta was observed. Purified recombinant GABP alpha and beta
interact with the HIV RafRE as judged from DNA binding assays.
Cotransfection experiments with GABP alpha and beta and Raf-1 kinase
demonstrate synergistic transactivation of the HIV-1 promoter. Point
mutations in the HIV RafRE abolished the Raf-1 kinase as well as GABP
alpha- and beta-induced transactivation. The observed Raf-1-GABP synergism
presumably involves phosphorylation of GABP subunits, as treatment of cells
with Raf-1 kinase activators serum and 12-O-tetradecanoylphorbol-13-acetate
increases phosphorylation of GABP in vivo. However, GABP is not a target of
Raf-1 kinase; instead, it is a substrate of mitogen-activated protein
kinase (MAPK/ERK), since in vitro phosphorylation of GABP alpha and beta
was achieved by the reconstituted protein kinase cascade but not with
purified Raf-1 or MEK. These results suggest that Raf-1 kinase- induced
activation of the HIV-1 promoter is mediated by the classical cytoplasmic
cascade resulting in MAPK/ERK-mediated phosphorylation of GABP alpha and
beta. Because the HIV RafRE corresponds to a region within the promoter
which is essential for regulation of HIV-1 expression, the data indicate
that in addition to NK-kappaB, GABP transcription factors are important for
induced expression of HIV.
Copyright © 1996, American Society for Microbiology
Raf-1 kinase targets GA-binding protein in transcriptional regulation of the human immunodeficiency virus type 1 promoter
Institute of Radiobiology and Cell Research, University of Wurzburg, Germany.
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