Stressful initiations
Paul Anderson* and
Nancy Kedersha
Division of Rheumatology, Immunology and Allergy, Brigham and Women's
Hospital, Smith 652, One Jimmy Fund Way, Boston, MA 02115, USA
*
Author for correspondence (e-mail:
panderson{at}rics.bwh.harvard.edu
)
 |
Summary
|
---|
Stress granules (SGs) are phase-dense particles that appear in the
cytoplasm of eukaryotic cells that have been exposed to environmental stress
(e.g. heat, oxidative conditions, hyperosmolarity and UV irradiation). SG
assembly is a consequence of abortive translational initiation: SGs appear
when translation is initiated in the absence of
eIF2-GTP-tRNAiMet, the ternary complex that normally
loads tRNAiMet onto the small ribosomal subunit.
Stress-induced depletion of eIF2-GTP-tRNAiMet allows the
related RNA-binding proteins TIA-1 and TIAR to promote the assembly of
eIF2-eIF5-deficient preinitiation complexes, the core constituents of SGs. The
mRNP components that make up the SG are in a dynamic equilibrium with
polysomes. As such, the SG appears to constitute a metabolic domain through
which mRNPs are continually routed and subjected to triage they are
first monitored for integrity and composition, and then sorted for productive
translational initiation or targeted degradation.
Key words: Stress granules, TIA-1, TIAR, Protein translation, eIF2alpha, Heat shock
 |
Introduction
|
---|
In response to environmental stress (e.g. heat, hyperosmolarity and
oxidative conditions), eukaryotic cells shut down protein synthesis in a
stereotypic response that conserves anabolic energy for the repair of
stress-induced damage. The translational arrest that accompanies environmental
stress is selective: whereas translation of constitutively expressed
`housekeeping' transcripts is turned off, translation of stress-induced
transcripts encoding heat shock proteins and some transcription factors is
maintained or enhanced. The stress-activated signaling cascades responsible
for reprogramming translation in stressed cells are becoming apparent. At the
apex of these signaling cascades is a family of serine/threonine kinases that
serve as sensors of environmental stress. Included in this family are the
following: (1)
PKR
, a
double-stranded RNA-dependent kinase that is activated by viral infection,
heat and UV irradiation (Williams,
2001
); (2) PERK/PEK, a resident ER protein that is activated when
unfolded proteins accumulate in the ER
(Harding et al., 2000
;
Patil and Walter, 2001
); (3)
GCN2, a protein that monitors amino acid levels in the cell and responds to
amino acid deprivation (Kimball,
2001
); and (4) HRI, a heme-regulated kinase that ensures the
balanced synthesis of globin chains and heme during erythroctye maturation
(Han et al., 2001
;
Lu et al., 2001
). Each of
these stress `sensors' phosphorylates eIF2
, a critical regulatory
component of the ternary complex (composed of eIF2
ß
bound
to tRNAiMet and GTP) that loads initiator
tRNAiMet onto the small ribosomal subunit to initiate
protein synthesis (Dever, 2002
;
Kimball, 2001
).
Phosphorylation of eIF2
converts the eIF2 ternary complex into a
competitive inhibitor of eIF2B, the GTP/GDP exchange factor that converts
inactive ternary complex (GDP-associated) to active ternary complex
(GTP-associated) (Krishnamoorthy et al.,
2001
). Thus, phosphorylation of eIF2
inhibits protein
translation by reducing the availability of the
eIF2-GTP-tRNAiMet ternary complex.
When translation is initiated in the absence of
eIF2-GTP-tRNAiMet, an eIF2/eIF5-deficient, `stalled'
48S* preinitiation complex is assembled
(Kedersha et al., 2002
). These
eIF2/eIF5-deficient preinitiation complexes and their associated mRNA
transcripts are dynamically routed to cytoplasmic foci known as stress
granules (SGs), in a process that requires the related RNA-binding proteins
TIA-1 and TIAR (Kedersha et al.,
2002
; Kedersha et al.,
2000
; Kedersha et al.,
1999
). In stressed cells, mRNA is in a dynamic equilibrium between
polysomes and SGs (Kedersha et al.,
2000
). Here we discuss the antagonistic roles of eIF2
and
TIA-1/TIAR in the assembly of polysomes and SGs in the context of a
translational checkpoint model, wherein TIA and eIF2 are functional
antagonists whose relative activities determine how many times a given
transcript is translated before it is degraded.
 |
Stress granules: a historical perspective
|
---|
When exposed to supra-ambient temperatures, eukaryotic cells exhibit
characteristic morphological changes in both nuclear and cytoplasmic
compartments. In the nucleus, actin filaments condense to form rod-shaped
bodies, and the granularity of nucleoli is altered
(Collier et al., 1988
;
Collier and Schlesinger, 1986
).
In the cytoplasm, the cytoskeleton is rearranged and the Golgi apparatus is
disrupted (Collier et al.,
1988
; Collier and Schlesinger,
1986
). The appearance of phase-dense cytoplasmic granules was
first noted in cultures of Peruvian tomato cells subjected to heat shock
(Nover et al., 1983
).
Low-molecular-weight heat shock proteins were identified as prominent
components of tomato heat shock granules
(Nover et al., 1983
).
Immunofluorescence microscopy revealed that they are also components of
phase-dense granules observed in cytoplasm of heat-shocked mammalian cells
(Arrigo et al., 1988
). The use
of immunofluorescence microscopy to detect stress-induced changes in the
subcellular localization of cellular components has allowed the identification
of cytoplasmic and nuclear microdomains that are assembled as part of the
normal stress response. Heat-shock-induced transcription factors have been
found to accumulate at discrete cytoplasmic and nuclear foci in response to
stress (Cotto et al., 1997
;
Cotto and Morimoto, 1999
;
Holmberg et al., 2000
;
Jolly et al., 2002
;
Scharf et al., 1998
).
Similarly, the subcellular localization of mRNP particles involved in various
aspects of mRNA metabolism is altered in cells subjected to environmental
stress (Gallouzi et al., 2000
;
Krebber et al., 1999
;
Michael et al., 1995
;
van der Houven van Oordt et al.,
2000
). Thus, immunofluorescence microscopy can be used to identify
subcellular domains that may regulate cellular metabolism during stress.
 |
Stress granules: the mRNA connection
|
---|
The identification of mRNA as a component of plant heat shock granules
provided a clue to the possible function of these cytoplasmic structures
(Nover et al., 1989
).
Remarkably, heat-shock-granule-associated mRNAs were found to encode
constitutively expressed `housekeeping' proteins but not newly synthesized
heat shock proteins (Nover et al.,
1989
). This result suggested that translationally silenced mRNAs
selectively accumulate at SGs, whereas translationally active mRNAs are
excluded. These mRNAs could still be translated in vitro and could also be
translated in vivo after the cells recovered from stress. Thus, the plant heat
shock granule was proposed to serve as a storage repository for untranslated
mRNAs.
Poly(A)+ RNA is also a component of mammalian SGs
(Kedersha et al., 1999
), as
demonstrated by in situ hybridization using oligo-dT
(Fig. 1). Recently,
sequence-specific in situ hybridization has shown that mRNA encoding inducible
HSP70 is selectively excluded from mammalian SGs (P.A. and N.K., unpublished).
Thus, the SGs found in the cytoplasm of both plant and animal cells are sites
at which untranslated mRNA accumulates in cells subjected to environmental
stress.

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Fig. 1. Assembly and disassembly of arsenite-induced stress granules. DU145 cells
were cultured in the absence (control) or presence of arsenite (1 mM) for 30
minutes (STRESS), washed and allowed to recover for 1 hour or 3 hours
(RECOVERY) before processing for two-color immunofluorescent microscopy. TIA-1
protein is identified using a polyclonal antibody (green). Poly(A)+ RNA is
revealed by in situ hybridation using an oligo-dT probe (red). Sites of
co-localization of TIA-1 and poly(A)+ RNA appear yellow. Nuclei are
counterstained using Hoechst dye (blue).
|
|
 |
Stress granules: composition of the mRNP
|
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Identification of the protein components of mammalian SGs was facilitated
by the discovery that the related RNA-binding proteins TIA-1 and TIAR are
robust markers of these cytoplasmic foci
(Kedersha et al., 1999
). TIA-1
and TIAR are concentrated in the nucleus of most cells, but heterokaryon
analysis reveals that both proteins continuously shuttle between the nucleus
and the cytoplasm (P.A. and N.K., unpublished). In response to environmental
stress, TIA-1 and TIAR accumulate in the cytoplasm, where they rapidly
aggregate to form mammalian stress granules
(Fig. 1)
(Kedersha et al., 1999
).
Following removal of a non-lethal stress, the SGs increase in size owing to
fusion of smaller SGs, and then rapidly disperse
[Fig. 1; see movie of this
process in Kedersha et al. (Kedersha et
al., 2000
)]. TIA-1 (Tian et
al., 1991
) and TIAR (Kawakami
et al., 1992
) possess three RNA-recognition motifs at their
N-termini and a glutamine-rich domain at their C-termini. TIA mutants lacking
the RNA-recognition motifs function as transdominant inibitors of SG assembly
in stressed cells (Kedersha et al.,
2000
) which suggests that the TIA proteins are required for SG
assembly.
The discovery of TIA-1 and TIAR as markers of SGs allowed the use of
dual-labeling studies to identify additional SG components. This analysis
revealed that components of small, but not large, ribosomal subunits
co-localize with TIA-1 and TIAR at SGs
(Kedersha et al., 2002
). The
absence of the large ribosomal subunit eliminated the possibility that SGs are
sites at which selected mRNAs are translated during stress. At the same time,
the presence of the small ribosomal subunit indicated that the mRNPs making up
SGs might be related to polysomes. Consistent with this premise is
compositional analysis revealing that many of the protein and RNA components
of SGs and polysomes are identical
(Kedersha et al., 2002
).
Translation is normally initiated when the small ribosomal subunit and its
associated initiation factors are recruited to a capped mRNA transcript to
form a 48S complex (Fig. 2,
left branch of pathway; green arrows). Hydrolysis of eIF2-associated GTP by
eIF5 displaces the early initiation factors, allowing the binding of the large
ribosomal subunit. Repeated cycles of successful initiation convert an mRNA
into a polysome. In stressed cells, activation of one or more eIF2
kinases (e.g. PKR, PERK/PEK, GCN2, HRI;
Fig. 2, red box) results in the
phosphorylation of eIF2
(Kimball,
2001
; Williams,
2001
), which consequently inhibits eIF2B, the GTP/GDP exchange
factor that charges the eIF2 ternary complex. The ensuing depletion of
eIF2-GTP-tRNAiMet prevents productive translation
initiation. Under these conditions, TIA-1 and TIAR promote the assembly of an
eIF2/eIF5-deficient preinitiation complex (denoted 48S* in
Fig. 2) that is routed to SGs
(Fig. 2, right branch of
pathway, red arrows) (Kedersha et al.,
2002
). RNA-binding proteins that either promote [human autoantigen
R (HuR) (Gallouzi et al.,
2000
)] or inhibit (tristetraprolin; P.A. and N.K., unpublished)
mRNA stability are also recruited to SGs. This suggests that the SG is a site
where the fates of specific transcripts are determined by the activity of
different RNA-binding proteins. Whether the SG is also a site of mRNA
processing (e.g. through degradation by exosomes) remains to be
determined.

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Fig. 2. Translational initiation in the absence or presence of stress. (Green
panels) In the absence of stress, eIF2B promotes the charging of the
eIF2-GTP-tRNAMet ternary complex by exchanging GDP for GTP. When
the eIF2-GTP-tRNAMet ternary complex is available, a canonical 48S
preinitiation complex is assembled at the 5' end of capped transcripts
(green arrow: Normal) and scanning begins. Upon recognition of the initiation
codon by the anticodon of tRNAMet, eIF5 promotes GTP hydrolysis,
and early initiation factors are displaced by the 60S ribosomal subunit. As
additional ribosomes are added to the transcript, the mRNA is converted into a
polysome. (Red panels) In stressed cells (red arrow: Stress), phosphorylation
of eIF2 by PKR, PERK, HRI or GCN2 converts eIF2 into a competitive
antagonist of eIF2B, depleting the stores of eIF2/GTP/tRNAMet.
Under these conditions, TIA-1 is included in a non-canonical,
eIF2/eIF5-deficient 48S* preinitiation complex (composed of all
components of the 48S pre-initiation complex except eIF2 and eIF5) that is
translationally silent. TIA-1 self-aggregation then promotes the accumulation
of these complexes at discrete cytoplasmic foci known as stress granules. Blue
square, eIF5; green triangle, eIF2 bound to GTP; yellow triangle, eIF2 bound
to GDP; red triangle, phospho-eIF2 bound to GDP.
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 |
Stress granules and polysomes: a dynamic equilibrium
|
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Earlier models suggested that heat-shock-granules are storage depots for
untranslated mRNAs, but mammalian SGs exhibit behavior inconsistent with such
a model. The antagonistic effects of different pharmacological inhibitors of
protein translation on SG assembly have revealed that SG-associated mRNA is in
a dynamic equilibrium with polysomes (Fig.
3) (Kedersha et al.,
2000
). Drugs that stabilize polysomes by freezing ribosomes on
translating mRNAs (e.g. cycloheximide and emetine) inhibit the assembly of SGs
and actively dissolve SGs in the continued presence of both stress and
eIF2
phosphorylation (Kedersha et
al., 2000
). Conversely, drugs that destabilize polysomes by
releasing ribosomes from mRNA transcripts (e.g. puromycin), promote the
assembly of SGs (Kedersha et al.,
2000
). This behavior suggested that SG-associated poly(A)+ RNA is
in equilibrium with polysomes. Direct evidence for the dynamic nature of SGs
was obtained in experiments using green fluorescent protein (GFP)-TIA-1 and a
construct in which GFP was fused to poly(A)-binding protein (PABP-GFP) to
monitor the assembly and disassembly of SGs in living cells
(Kedersha et al., 2000
). In
response to arsenite-induced oxidative stress, GFP-TIA-1 rapidly moves from
the nucleus to the cytoplasm, where it is evenly and diffusely distributed.
After
10 minutes, the cytoplasmic GFP-TIA-1 aggregates into discrete foci
that coalesce and slowly enlarge over the next 20 minutes. When arsenite is
washed out of the cells, the SGs slowly disassemble with similar kinetics.
However, this slow and steady accumulation of GFP-TIA-1 at SGs is misleading.
Fluorescence recovery after photobleaching (FRAP) analysis reveals that
GFP-TIA-1 shuttles in and out of SGs very rapidly such that 50% of
SG-associated GFP-TIA-1 is replaced every 2 seconds. Although the rate at
which mRNA shuttles in and out of SGs was not determined directly, GFP-PABP
was used as a surrogate marker for its associated mRNA. Interestingly,
GFP-PABP shuttles in and out of SGs at a rate that is ten times slower than
that of GFP-TIA-1 (i.e. 50% of SG-associated GFP-PABP is replaced every 20
seconds). Given these kinetics, and considering that the dominant negative
mutant of TIA-1 (e.g. TIA-1
RRM) prevents SG assembly altogether
(Kedersha et al., 2000
), it
appears that TIA-1 actively escorts untranslated mRNA to SGs. These data
reveal that SGs are highly dynamic structures despite their apparent stability
in real-time microscopy.
The dynamic movement of mRNA into and out of SGs argues against a model in
which SGs are passive repositories of untranslated mRNAs that accumulate in
stressed cells. Rather, the SG is more likely to serve as a `way station'
through which untranslated mRNAs pass before being translated or degraded
(Kedersha et al., 2000
). Taken
together, the data suggest that TIA-1 and TIAR act downstream of the
stress-induced phosphorylation of eIF2
to drive mRNA from polysomes to
SGs (Fig. 3). The central
importance of the PKR/eIF2
pathway in the cellular response to stress
is indicated by the number of eukaryotic viruses that must disable PKR or
reverse the phosphorylation of eIF2
to effect productive infection
(Barber, 2001
). If TIA-1 is an
important component of the PKR/eIF2 stress response pathway, viruses might
target TIA-1 to subvert this protective response. Indeed, TIA-1 and TIAR have
been found to bind to specific sequences encoded by both West Nile Virus RNA
(W. Li, Y. Li, N.K. et al., unpublished) and Sendai virus RNA (F. Iseni, D.
Barcin, M. Nishhio et al., unpublished).
Fig. 3 depicts the dynamic
equilibrium between polysomes and SGs and summarizes the molecular pathways
that regulate this equilibrium.
 |
Translational triage model
|
---|
Fig. 4 shows a model
depicting the molecular stages in the conversion of a polysome into an SG and
the subsequent fate of its component mRNAs. The formation of an eIF2-deficient
48S* complex at the 5' end of the polysomal mRNA allows TIA
to bind to RNA in lieu of ternary complex (stage 1), resulting in an abortive
initiation event. This allows the previously initiated ribosomes to run off
the transcript (stages 2 and 3) until only the 48S* complex remains
bound to the 5' end of the RNA (stage 4). Should an eIF5-eIF2-containing
ternary complex become available at any time during this process, the
transcript can still be reinitiated (green arrow) by displacement of TIA with
eIF5-eIF2-GTP-tRNAiMet. However, as stress conditions
disrupt TIA shuttling such that the normally nuclear TIA accumulates in the
cytoplasm, more TIA becomes available to bind nonspecifically to the mRNA. The
self-aggregation of TIA drives SG assembly (stages 5 and 6). Other RNA-binding
proteins such as HuR and tristetraprolin (TTP) are recruited to the SG to
determine the fate of specific transcripts. ATP depletion promotes the
assembly of SGs without increasing phospho-eIF2
(Kedersha et al., 2002
), and
overexpression of HSP70 results in the dispersal of the prion-like domain of
TIA-1 in vivo (P.A. and N.K., unpublished). Therefore, we propose that HSP70
and ATP are required to extricate mRNAs from the SG, probably by altering the
conformation of the TIA prion-like domain. Thus, according to this model,
phospho-eIF2
initiates the formation of SGs, a sudden flood of
untranslated polyA(+) mRNA is released from polysomes, TIA proteins aggregate
this RNA, and HSP70 (and ATP) is required to dis-aggregate the TIA
proteins.

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Fig. 4. Proposed mechanism for the assembly of stress granules. Assembly of an
eIF2/eIF5-deficient preinitiation complex at the 5' end of a polysome
results in translational arrest (1). As elongating ribosomes `run-off' the
mRNA (2,3), the polysome is converted into a 48S* preinitiation
complex (4) that is routed by TIA-1 into stress granules (5,6) or productively
initiated by the replacement of TIA with eIF5/eIF2/GTP/tRNAmet
(green circle). A requirement for HSP 70 and ATP in removing mRNAs from the SG
is indicated. Destabilizing elements (red) such as tristetraprolin (TTP) are
proposed to direct selected stress granule mRNAs to sites of degradation,
whereas stabilizing elements such as HuR (blue) are proposed to direct
selected mRNAs to sites of storage and/or reinitiation. By this triage
process, the SG may monitor the structure and integrity of mRNP complexes and
determine the fate of specific RNAs.
|
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 |
Translational silencing without visible stress granules
|
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Overexpression of recombinant TIA-1 represses the production of
co-expressed reporter genes in the absence of exogenous stress
(Kedersha et al., 2000
). It is
therefore likely that some eIF2/eIF5-deficient `abortive' preinitiation
complexes are assembled under normal conditions, and that their frequency
determines how often mRNA transcripts are subjected to triage. Consistent with
this view, endogenous TIA-1 and TIAR repress the translation of TNF
transcripts in the absence of stress. Both TIA-1 and TIAR are components of a
regulatory complex that binds to an adenine/uridine-rich element found in the
3' untranslated region of TNF
transcripts
(Gueydan et al., 1999
;
Piecyk et al., 2000
).
LPS-activated macrophages derived from mice lacking either TIA-1 or TIAR
overexpress TNF
compared with wild-type controls. In macrophages
lacking TIA-1, the polysome profile of TNF
transcripts is shifted such
that the percentage of TNF
transcripts associated with polysomes is
increased compared with that of wild-type macrophages
(Piecyk et al., 2000
). This
suggests that TIA-1 represses the translation of TNF
by promoting the
assembly of non-polysomal mRNP complexes. Sucrose-density-gradient-analysis
reveals that TIA-1 is found in low-density fractions that contain soluble
proteins, as well as higher density fractions that contain 40-60S mRNPs, but
not in polysomes (Kedersha et al.,
2000
). It is likely that the TIA-mediated shift of TNF-
mRNA transcripts away from polysomes occurs through the assembly of abortive
eIF2/eIF5-deficient pre-initiation complexes that are similar or identical to
those that comprise the core units of SGs.
 |
Nuclear history
|
---|
Their visible roles in the cytoplasm notwithstanding, TIA-1 and TIAR
normally predominate in the nucleus, where they have been shown to act as
selective regulators of alternative mRNA splicing
(Del Gatto-Konczak et al.,
2000
; Forch et al.,
2000
; Le Guiner et al.,
2001
). The binding of either TIA-1 or TIAR to uridine-rich
elements found in intronic sequences located downstream of weak 5'
splice sites promotes the recruitment of U1snRNP and the inclusion of
`cryptic' alternative exons that are otherwise excised from the heteronuclear
RNA. In their dual ability to regulate both mRNA splicing and translation,
TIA-1 and TIAR resemble several other multifunctional RNA-binding proteins,
including PTB, CUB-BP-related proteins, La, hnRNPK and hnRNP A1
(Wilkinson and Shyu, 2001
;
Ladomery, 1997
). The process
of mRNA splicing has been shown to `mark' selected mRNA transcripts for both
quality control in the nucleus and translational control in the cytoplasm
(Le Hir et al., 2000a
;
Le Hir et al., 2000b
). It is
therefore possible that heteronuclear mRNAs encoding introns that are
recognized by TIA-1/TIAR retain these proteins at the exon-exon junction
following the removal of the intron upon splicing. Upon arrival in the
cytoplasm, transcripts that are `marked' by TIA-1/TIAR could be selectively
regulated at the level of mRNA stability or translatability. Such an
explanation would allow for the nuclear loading of TIA-1 and TIAR onto
selected transcripts that become subject to translational silencing once in
the cytoplasm. It also suggests a mechanism whereby stress-induced transcripts
might be excluded from SGs: mRNAs transcribed during stress would escape being
marked by TIA proteins, whose normal shuttling appears to be disrupted when
they are routed into cytoplasmic SGs.
 |
Conclusions/perspectives
|
---|
Molecular modifications (particularly phosphorylation) are widely
appreciated to regulate many distinct steps of the translational initiation
process (Clemens, 2001
;
Lawrence and Brunn, 2001
;
Mahalingam and Cooper, 2001
).
The importance of subcellular compartmentalization of translational components
is an important mode of regulation that occurs in living cells but is less
understood. Key components of the translational apparatus (e.g. mRNA and its
associated proteins, ribosomal subunits and translation initiation factors)
move between the nucleus and the cytoplasm in a regulated manner. The
localized availability and hence the activity of these components is regulated
by interactions with organelles and with the cytoskeleton. The rapid assembly
and disassembly of mammalian SGs is a striking illustration of this type of
regulation. SGs coalesce into existence as mRNAs are released from polysomes
during stress-induced translational arrest and melt away like snowflakes as
the cell adapts or recovers from the stress.
The molecular trigger for SG formation is the abortive translation that
occurs when a transcript is initiated without the eIF2-GTP-tRNAMet
ternary complex. The assembly of translationally inactive initiation complexes
lacking eIF2, coupled with increased levels of cytoplasmic TIA, allows the
RNA-binding proteins TIA-1 and/or TIAR to redirect untranslated mRNAs from
polyribosomes to SGs (Fig. 4).
Thus, TIA-1/TIAR and eIF2-GTP-tRNAiMet appear to act as
antagonists that regulate the equilibrium between polysomes and SGs (Figs
3,
4). It remains to be determined
whether TIA-1 and eIF2-GTP-tRNAMet compete for binding to a common
site on the preinitiation complex. Regardless of the details of molecular
mechanism, the functional antagonism between eIF2 and TIA may determine the
frequency with which a given mRNA transcript is initiated before being subject
to a checkpoint at which mRNP structure and composition is monitored. If the
ratio of TIA-1/TIAR to eIF2-GTP-tRNAMet were 1:10, one might
predict that, on average, ten productive initiation events would occur before
a TIA-1/TIAR-containing translationally incompetent pre-initiation complex is
assembled. As translating ribosomes `run off' this mRNA, the
eIF2/eIF5-deficient complex would be directed to an SG, where the integrity
and composition of the mRNP might determine whether the transcript is
reinitiated, degraded or packaged into an untranslated mRNP particle.
Although considerable progress has been made in elucidating the
relationship between SGs and translational initiation, we have much to learn
about the connection between SGs and molecular chaperones. The aggregation
domain of TIA is its glutamine-rich C-terminus, which resembles prion protein,
and is essential for SG assembly. Specific molecular chaperones are required
to disperse aggregated TIA-1 in living cells (P.A. and N.K., unpublished).
These chaperones are also required for the adaptive response to stress, which
suggests that SGs constitute a point of crosstalk between molecular chaperones
and eIF2 kinases. Do SGs function as signal transduction domains? And what
else is in SGs? Plant SGs contain not only RNA and small HSPs but also heat
shock transcription factors. Do mammalian SGs also contain stress-related
transcription factors and, if so, which ones? The end of stress is not yet in
sight!
 |
Footnotes
|
---|
eIF, eukaryotic initiation factor; GCN2, general control nonrepressed 2;
HRI, heme-regulated inhibitor; PERK, PKR-like endoplasmic reticulum kinase;
PKR, double-stranded RNA-dependent protein kinase; RRM, RNA-recognition motif;
TIA-1, T-cell intracellular antigen-1; TIAR, T-cell intracellular
antigen-related. 
 |
References
|
---|
Arrigo, A. P., Suhan, J. P. and Welch, W. J.
(1988). Dynamic changes in the structure and intracellular locale
of the mammalian low-molecular-weight heat shock protein. Mol.
Cell. Biol. 8,5059
-5071.[Medline]
Barber, G. N. (2001). Host defense, viruses and
apoptosis. Cell Death Differ.
8, 113-126.[Medline]
Clemens, M. J. (2001). Initiation factor eIF2
alpha phosphorylation in stress responses and apoptosis. Prog. Mol.
Subcell. Biol. 27,57
-89.[Medline]
Collier, N. C. and Schlesinger, M. J. (1986).
The dynamic state of heat shock proteins in chicken embryo fibroblasts.
J. Cell Biol. 103,1495
-1507.[Abstract]
Collier, N. C., Heuser, J., Levy, M. A. and Schlesinger, M.
J. (1988). Ultrastructural and biochemical analysis of the
stress granule in chicken embryo fibroblasts. J. Cell
Biol. 106,1131
-1139.[Abstract]
Cotto, J. J. and Morimoto, R. I. (1999).
Stress-induced activation of the heat-shock response: cell and molecular
biology of heat-shock factors. Biochem. Soc. Symp.
64,105
-118.[Medline]
Cotto, J., Fox, S. and Morimoto, R. (1997).
HSF1 granules: a novel stress-induced nuclear compartment of human cells.
J. Cell Sci. 110,2925
-2934.[Abstract/Free Full Text]
Del Gatto-Konczak, F., Bourgeois, C. F., le Guiner, C., Kister,
L., Gesnel, M. C., Stevenin, J. and Breathnach, R. (2000).
The RNA-binding protein TIA-1 is a novel mammalian splicing regulator acting
through intron sequences adjacent to a 5' splice site. Mol.
Cell. Biol. 20,6287
-6299.[Abstract/Free Full Text]
Dever, T. E. (2002). Gene-specific regulation
by general translation factors. Cell
108,545
-556.[Medline]
Forch, P., Puig, O., Kedersha, N., Martinez, C., Granneman, S.,
Seraphin, B., Anderson, P. and Valcarcel, J. (2000). The
apoptosis-promoting factor TIA-1 is a regulator of alternative pre- mRNA
splicing. Mol. Cell 6,1089
-1098.[Medline]
Gallouzi, I. E., Brennan, C. M., Stenberg, M. G., Swanson, M.
S., Eversole, A., Maizels, N. and Steitz, J. A. (2000). HuR
binding to cytoplasmic mRNA is perturbed by heat shock. Proc. Natl.
Acad. Sci. USA 97,3073
-3078.[Abstract/Free Full Text]
Gueydan, C., Droogmans, L., Chalon, P., Huez, G., Caput, D. and
Kruys, V. (1999). Identification of TIAR as a protein binding
to the translational regulatory AU-rich element of tumor necrosis factor
mRNA. J. Biol. Chem.
274,2322
-2326.[Abstract/Free Full Text]
Han, A. P., Yu, C., Lu, L., Fujiwara, Y., Browne, C., Chin, G.,
Fleming, M., Leboulch, P., Orkin, S. H. and Chen, J. J.
(2001). Heme-regulated eIF2alpha kinase (HRI) is required for
translational regulation and survival of erythroid precursors in iron
deficiency. EMBO J. 20,6909
-6918.[Abstract/Free Full Text]
Harding, H. P., Novoa, I., Zhang, Y., Zeng, H., Wek, R.,
Schapira, M. and Ron, D. (2000). Regulated translation
initiation controls stress-induced gene expression in mammalian cells.
Mol. Cell 6,1099
-1108.[Medline]
Holmberg, C. I., Illman, S. A., Kallio, M., Mikhailov, A. and
Sistonen, L. (2000). Formation of nuclear HSF1 granules
varies depending on stress stimuli. Cell Stress
Chaperones 5,219
-228.[Medline]
Jolly, C., Konecny, L., Grady, D. L., Kutskova, Y. A., Cotto, J.
J., Morimoto, R. I. and Vourc'h, C. (2002). In vivo binding
of active heat shock transcription factor 1 to human chromosome 9
heterochromatin during stress. J. Cell Biol.
156,775
-781.[Abstract/Free Full Text]
Kawakami, A., Tian, Q., Duan, X., Streuli, M., Schlossman, S. F.
and Anderson, P (1992). Identification and functional
characterization of a TIA-1-related nucleolysin. Proc. Natl. Acad.
Sci. USA 89,8681
-8685.[Abstract]
Kedersha, N. L., Gupta, M., Li, W., Miller, I. and Anderson,
P. (1999). RNA-binding proteins TIA-1 and TIAR link the
phosphorylation of eIF-2 alpha to the assembly of mammalian stress granules.
J. Cell Biol. 147,1431
-1442.[Abstract/Free Full Text]
Kedersha, N., Cho, M., Li, W., Yacono, P., Chen, S., Gilks, N.,
Golan, D. and Anderson, P. (2000). Dynamic shuttling of TIA-1
accompanies the recruitment of mRNA to mammalian stress granules.
J. Cell Biol. 151,1257
-1268.[Abstract/Free Full Text]
Kedersha, N., Chen, S., Gilks, N., Li, W., Miller, I., Stahl, J.
and Anderson, P. (2002). Evidence that ternary complex
(eIF2-GTP-tRNAMet)-deficient preinitiation complexes are core constituents of
mammalian stress granules. Mol. Biol. Cell
13,195
-210.[Abstract/Free Full Text]
Kimball, S. R. (2001). Regulation of
translation initiation by amino acids in eukaryotic cells. Prog.
Mol. Subcell. Biol. 26,155
-184.[Medline]
Krebber, H., Taura, T., Lee, M. S. and Silver, P. A.
(1999). Uncoupling of the hnRNP Np13p from mRNAs during the
stress-induced block in mRNA export. Genes Dev
13,1994
-2004.[Abstract/Free Full Text]
Krishnamoorthy, T., Pavitt, G. D., Zhang, F., Dever, T. E. and
Hinnebusch, A. G. (2001). Tight binding of the phosphorylated
alpha subunit of initiation factor 2 (eIF2alpha) to the regulatory subunits of
guanine nucleotide exchange factor eIF2B is required for inhibition of
translation initiation. Mol. Cell. Biol.
21,5018
-5030.[Abstract/Free Full Text]
Ladomery, M. (1997). Multifunctional proteins
suggest connections between transcriptional and post-transcriptional
processes. Bioessays 19,903
-909.[Medline]
Lawrence, J. C., Jr and Brunn, G. J. (2001).
Insulin signaling and the control of PHAS-I phosphorylation. Prog.
Mol. Subcell. Biol. 26,1
-31.[Medline]
Le Guiner, C., Lejeune, F., Galiana, D., Kister, L., Breathnach,
R., Stevenin, J. and del Gatto-Konczak, F. (2001). TIA-1 and
TIAR activate splicing of alternative exons with weak 5' splice sites
followed by a U-rich stretch on their own pre-mRNAs. J. Biol.
Chem. 276,40638
-40646.[Abstract/Free Full Text]
Le Hir, H., Izaurralde, E., Maquat, L. E. and Moore, M. J.
(2000a). The spliceosome deposits multiple proteins 20-24
nucleotides upstream of mRNA exon-exon junctions. EMBO
J. 19,6860
-6869.[Abstract/Free Full Text]
Le Hir, H., Moore, M. J. and Maquat, L. E.
(2000b). Pre-mRNA splicing alters mRNP composition: evidence for
stable association of proteins at exon-exon junctions. Genes
Dev. 14,1098
-1108.[Abstract/Free Full Text]
Lu, L., Han, A. P. and Chen, J. J. (2001).
Translation initiation control by heme-regulated eukaryotic initiation factor
2alpha kinase in erythroid cells under cytoplasmic stresses. Mol.
Cell. Biol. 21,7971
-7980.[Abstract/Free Full Text]
Mahalingam, M. and Cooper, J. A. (2001).
Phosphorylation of mammalian eIF4E by Mnk1 and Mnk2: tantalizing prospects for
a role in translation. Prog. Mol. Subcell. Biol.
27,132
-142.[Medline]
Michael, M. W., Choi, M. and Dreyfuss, G.
(1995). A nuclear export signal in hnRNP A1: A signal-mediated,
temperature-dependent nuclear protein export pathway.
Cell 83,415
-422.[Medline]
Nover, L., Scharf, K. D. and Neumann, D.
(1983). Formation of cytoplasmic heat shock granules in tomato
cell cultures and leaves. Mol. Cell. Biol.
3,1648
-1655.[Medline]
Nover, L., Scharf, K. and Neumann, D. (1989).
Cytoplasmic heat shock granules are formed from precursor particles and are
associated with a specific set of mRNAs. Mol. Cell.
Biol. 9,1298
-1308.[Medline]
Patil, C. and Walter, P. (2001). Intracellular
signaling from the endoplasmic reticulum to the nucleus: the unfolded protein
response in yeast and mammals. Curr. Opin. Cell Biol.
13,349
-355.[Medline]
Piecyk, M., Wax, S., Beck, A., Kedersha, N., Gupta, M., Maritim,
B., Chen, S., Gueydan, C., Kruys, V., Streuli, M. and Anderson, P.
(2000). TIA-1 is a translational silencer that selectively
regulates the expression of TNF-alpha. EMBO J.
19,4154
-4163.[Abstract/Free Full Text]
Scharf, K. D., Heider, H., Hohfeld, I., Lyck, R., Schmidt, E.
and Nover, L. (1998). The tomato Hsf system: HsfA2 needs
interaction with HsfA1 for efficient nuclear import and may be localized in
cytoplasmic heat stress granules. Mol. Cell. Biol.
18,2240
-2251.[Abstract/Free Full Text]
Tian, Q., Streuli, M., Saito, H., Schlossman, S. F. and
Anderson, P. (1991). A polyadenylate binding protein
localized to the granules of cytolytic lymphocytes induces DNA fragmentation
in target cells. Cell
67,629
-639.[Medline]
van der Houven van Oordt, W., Diaz-Meco, M. T., Lozano, J.,
Krainer, A. R., Moscat, J. and Caceres, J. F. (2000). The
MKK(3/6)-p38-signaling cascade alters the subcellular distribution of hnRNP A1
and modulates alternative splicing regulation. J. Cell
Biol. 149,307
-316.[Abstract/Free Full Text]
Wilkinson, M. F. and Shyu, A. B. (2001).
Multifunctional regulatory proteins that control gene expression in both the
nucleus and the cytoplasm. Bioessays
23,775
-787.[Medline]
Williams, B. R. (2001). Signal integration via
PKR. Sci. STKE 2001,RE2
.[Medline]
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