REGULATION OF ACTIVATION OF GCN2, A STRESS SENSOR IN
MAMMALS.
Pereira, C. M.1; Delamano, A.1; Roffe, M.1; Jaqueta, C. B.2; Mello, L. E.2; Wek,
R.3; Castilho, B. A.1. 1 Departamento de Microbiologia, Imunologia e
Parasitologia, 2 Departamento de Fisiologia, Universidade Federal de São
Paulo, São Paulo, SP, 04023-062, Brasil; 3 Department of Biochemistry and
Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana
46202, USA.
Translational control induced by phosphorylation of the alpha subunit of the
eukaryotic initiation factor 2 (eIF2) is important for cellular adaptation to
environmental stresses, and plays a pivotal role in mammalian physiology. Its
imbalance leads to several pathological conditions. GCN2, one of the four
kinases that phosphorylate eIF2 in mammals, controls protein synthesis and the
induction of a complex gene regulatory network, including ATF4 (CREB-2) and
CHOP expression, in response to indispensable amino acid starvation. GCN2
has been implicated in regulating feeding behavior and learning and memory in
mice. Moreover, GCN2 is activated by UV stress and proteasome inhibition. This
kinase is a multi-domain protein with a protein kinase domain juxtaposed to
sequences homologous to histidyl-tRNA synthetase enzymes (HisRS).
Uncharged tRNA that accumulates during amino acid starvation binds to the
HisRS-related domain of GCN2, contributing to an altered conformation that
enhances GCN2 phosphorylation of eIF2. At the N-terminus of GCN2 is a socalled GI domain that interacts with GCN1, a ribosome- associated protein that is
proposed to facilitate the transport of uncharged tRNA to the HisRS-related
region of GCN2. In the yeast model Saccharomyces cerevisiae, activation of
GCN2 requires its physical interaction with GCN1, and its disruption via
alterations in GCN2 or GCN1 leads to reduced GCN2 kinase activity. We show
here that in mammals GCN1 also associates with GCN2, as determined by
immunoprecipitation assays in MEF cells. Furthermore, overexpression of the
segment of GCN1 that binds to GCN2 has a dominant negative phenotype, with
the cells less responsive to diverse stress conditions. Thus, the GCN2-GCN1
interaction is required for activation of GCN2 under stress conditions in
mammals. Interestingly, we show in this work that the protein IMPACT can
function as a GCN2 repressor, inhibiting the kinase activation in cells under a
variety of stress conditions. We hypothesize that IMPACT competes with GCN2
for association with GCN1. Displacement of GCN1 from GCN2 would lead to an
inability to activate this eIF2 kinase and to impaired translational control. IMPACT
is predominantly and differentially expressed in the brain, being highly abundant
in hypothalamic and brain stem neurons. On the other hand, GCN2 and GCN1
are expressed at the same levels in all organs and brain parts. We thus propose
that the activation of GCN2 can be differentially controlled in the brain by the
presence or absence of IMPACT in different neuronal groups. Thus, GCN2
activity and its regulation by IMPACT are central to stress pathways operating in
the CNS.
Supported by FAPESP.