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Mol. Cell. Biol., 08 1995, 4497-4506, Vol 15, No. 8
SA Wek, S Zhu and RC Wek
Protein kinase GCN2 is a multidomain protein that contains a region
homologous to histidyl-tRNA synthetases juxtaposed to the kinase catalytic
moiety. Previous studies have shown that in response to histidine
starvation, GCN2 phosphorylates eukaryotic initiation factor 2 (eIF-2), to
induce the translational expression of GCN4, a transcriptional activator of
genes subject to the general amino acid control. It was proposed that the
synthetase-related sequences of GCN2 stimulate the activity of the kinase
by interacting directly with uncharged tRNA that accumulates during amino
acid limitation. In addition to histidine starvation, expression of GCN4 is
also regulated by a number of other amino acid limitations. Questions that
we posed in this report are whether uncharged tRNA is the most direct
regulator of GCN2 and whether the function of this kinase is required to
recognize each of the different amino acid starvation signals. We show that
GCN2 phosphorylation of eIF-2, and the resulting general amino acid control
pathway, is stimulated in response to starvation for each of several
different amino acids, in addition to histidine limitation. Cells
containing a defective aminoacyl-tRNA synthetase also stimulated GCN2
phosphorylation of eIF-2 in the absence of amino acid starvation,
indicating that uncharged tRNA levels are the most direct regulator of GCN2
kinase. Using a Northwestern blot (RNA binding) assay, we show that
uncharged tRNA can bind to the synthetase-related domain of GCN2. Mutations
in the motif 2 sequence conserved among class II synthetases, including
histidyl-tRNA synthetases, impair the ability of this synthetase-related
domain to bind tRNA and abolish GCN2 phosphorylation of eIF-2 required to
stimulate the general amino acid control response. These in vivo and in
vitro experiments indicate that synthetase-related sequences regulate GCN2
kinase function by monitoring the levels of multiple uncharged tRNAs that
accumulate during amino acid limitations.
Copyright © 1995, American Society for Microbiology
The histidyl-tRNA synthetase-related sequence in the eIF-2 alpha protein kinase GCN2 interacts with tRNA and is required for activation in response to starvation for different amino acids
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis 46202-5122, USA.
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