Table Of ContentProgress in Molecular and 18
Subcellular Biology
Series Editors
Ph. Jeanteur, Y. Kuchino,
W.E.G. Muller (Managing Editor)
P.L. Paine
Springer
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Ph. Jeanteur (Ed.)
Cytoplasmic fate
of messenger RNA
With 25 Figures
, Springer
Prof. Dr. PHILIPPE JEANTEUR
Institut de Genetique Moleculaire
C.N.R.S., B.P. 5051
1919 route de Mende
34033 Montpellier Cedex 01
France
ISBN-13: 978-3-642-64420-7 Springer-Verlag Berlin Heidelberg New York
Library of Congress Cataloging-in· Publication Data. Cytoplasmic fate of messenger RNAj
Ph. Jeanteur, ed. p. em. - (Progress in molecular and subcellular biology: 18) Includes
bibliographical references and index.
ISBN-13: 978-3-642-64420-7 e-ISBN-13: 978-3-642-60471-3
001: 10.1007/978-3-642-60471-3
I. Messenger RNA - Metabolism.
2. Cytoplasm. I. Jeanteur, Ph. (Philippe) II. Series. QH506.P76 no. 18 [QP623.5.M47]
574.87'3283-dc20 97-25885
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Preface
Among all cellular RNA species of the three main types, ribosomal
RNA, transfer RNA or messenger RNA, be they from prokaryotic
or eukaryotic organisms, the prokaryotic mRNA is unique in that it
has no precursor and is synthesized in the same mature form as it is
translated into proteins. In fact, ribosomes join the nascent mRNA
chain and engage in protein synthesis long before its transcription is
complete. Provisions are even made for slowing down the ribo
somes at some sites to prevent them from catching up with the
RNA-polymerase. Of course, such a situation is only possible in the
prokaryotic world where there is no such thing as a nuclear mem
brane physically secluding the transcription process from the cy
toplasm where translation is restricted.
Quite in the opposite extreme, the eukaryotic pre-messenger
RNA has to suffer many and sometimes drastic steps of maturation
(capping, polyadenylation, splicing, edition) before the decision is
made to export it to the cytoplasm. That is where it enters the scope
of this book. Once in the cytoplasm, many options are still open to
it: its entrance into polysomes may be delayed (as it is in unfertilized
eggs) or merely prohibited (ferritin mRNA in iron-starved cells),
directed to specific locations within the cytoplasm or be more or less
rapidly degraded.
During gametogenesis and early development, translational
control is probably the most significant level of gene expression.
Most mRNAs are in a translationnally repressed state in oocytes
(Osborne and Richter, Amaldi and Pierandrei-Amaldi) and im
mediately engage in translation after fertilization. The role of
polyadenylation in this process as well as that of deadenylation in
the subsequent decay of these mRNAs is addressed by Osborne and
Richter who emphasize that the complexity of the cis-acting RNA
sequence elements involved reflects their capacity for regulation as
opposed to the relative simplicity of nuclear polyadenylation sig
nals. Although the signals for cytoplasmic polyadenylation are
beginning to emerge, those for deadenylation are much less clear.
Much also remains to be learned about the biochemistry of cyto
plasmic polyadenylation and deadenylation and this issue is ad-
VI Preface
dressed by Virtanen and Astrom. The general question of how
polyadenylation contributes to translational arrest and whether it is
a harbinger for degradation remains largely open.
Another set of genes where translational control is a major
regulatory level is that for ribosomal proteins (Amaldi and Pier
andrei-Amaldi). In addition to being also transcriptionally regu
lated, most, if not all of these genes resort to translational control to
precisely match their production to their stoichiometry in the ri
bosome. This takes place not only after fertilization but also in
cultured cells. mRNAs for these genes contain a 5' Terminal Oligo
Pyrimidine tract and are therefore referred to as TOP mRNAs.
Amaldi and Pierandrei-Amaldi address the problem of how this
sequence controls their translation by shifting them between free
and polysome-bound forms.
A quite elegant example of translation regulation in the devel
oping drosophila embryo has very recently been discovered (Dub
nau and Struhl 1996; Rivera-Pomar et al. 1996).1 The homeo
domain of bicoid binds to the 3' UnTranslated Region (UTR) of
caudal mRNA and represses its translation, resulting in opposite
antero-posterior gradients of these two proteins.
The regulation of iron-controlled proteins provides the best
understood example of a mechanism linking RNA-protein inter
actions to translational regulation in the case of ferritin and to its
degradation in the case of the transferrin receptor. These issues are
reviewed in two chapters (Henderson and Kuhn; Muckenthaler and
Hentze). These regulations are mediated by RNA-protein interac
tions between Iron Response Elements (IRE) and Iron Response
Proteins (IRP). The characteristics of these interactions and the
biochemistry of IRP-l and IRP-2 are the focus of the review by
Henderson and Kuhn which also emphasizes the link between the
RNA-binding and enzymatic properties of these proteins. The
chapter by Muckenthaler and Hentze is more concerned with the
mechanism by which these RNA-protein complexes interfere with
mRNA translation or degradation. Binding of IRPs to the IRE
located in the cap proximal part of ferritin mRNA 5'UTR pre
vents its association with the 43S translation pre-initiation complex.
Both chapters suggest that the two IRPs which have different but
overlapping specificities might have other, still unknown, mRNA
targets (Henderson and Kuhn) and more generally that the same
type of RNA-protein interaction but involving different partners
might operate in situations other than the regulation of iron me-
'Dubnau J, Struhl G (1996) Nature 379:694-699. River-Pomar R, Niessing D,
Schmidt-Ott U, Gehring WJ, and Jackie H (1996) Nature 379:746-749.
Preface VII
tabolism, for example, during early development (Muckenthaler
and Hentze).
mRNA degradation as a major level of control for at least some
sets of genes in somatic cells is becoming more and more widely
recognized but probably not yet fully appreciated. Rapid de
gradation of mRNA is a requisite for the control of some categories
of genes (for example, nuclear oncogenes, cytokines, inflammatory
proteins) whose expression needs to be transient. The delineation of
the RNA sequence elements which target them for regulated de
gradation is addressed in several of the following chapters. The best
characterized among these sequences are the AU-rich elements
(ARE) located in the 3'UTR of unstable mRNAs. Jarzembowski
and Malter describe the identification and characterization of AU
binding proteins, while the development of cell-free systems for
analyzing cytoplasmic mRNA turnover is dealt with by De Maria
and Brewer.
The issue of nucleases which contribute to the degradation of
mRNA is addressed by two contributions. The chapter by Virtanen
and Astrom first reviews the biochemistry of polyadenylation and
the enzymology of polyA shortening with respect to the specific 3'
polyA exoribonucleases present in various eukaryotic tissues. It also
examines the functional significance of polyA in translation and of
its regulated shortening in mRNA decay. Starting from the 2-SA
activated RNAse L, which is induced by interferons, Bisbal goes on
to speculate that this activatable system might represent the mod
ulation of an otherwise physiological activity required for normal
mRNA metabolism.
Finally, Veyrune et al. first discuss the link between ARE,
translation, and degradation. They also elaborate on the possibility
that directing some mRNAs to specific subcellular locations can
contribute to their regulation. For example, it can be speculated
that the unusual perinuclear localization of c-myc mRNA can be of
functional relevance. This localization appears to depend on the
3'UTR. Whether and how it is connected to the ARE degradation
signals remain to be elucidated.
Transcriptional control remains obviously an essential basis for
general gene regulation in eukaryotes mostly when it amounts to
switching them on, but this is eventually achieved only after a long
cascade of events have taken place. The point of this book is to
provide evidence that this control is far from accounting for all the
facets of gene regulation. Cytoplasmic control is required for the
immediate and massive induction of protein synthesis in early egg
development. Conversely, when it comes to rapidly switching off
specific genes, efficient and targeted mRNA degradation must be
called upon. The very existence of these cytoplasmic levels of reg-
VIII Preface
ulation has only been recognized in recent years. Although they
have now reached a status of evidence, it is my expectation that we
are still far from fully appreciating the contribution of the RNA
itself to the fine and immediate tuning of gene expression.
France, November 1996 PH. J EANTEUR
Contents
TOP Genes: A Translationally Controlled Class of Genes
Including Those Coding for Ribosomal Proteins . . . . . . . . . .
F. AMALDI and P. PIERANDREI-AMALDI
Introduction ............................. .
2 TOP Genes and TOP mRNAs . . . . . . . . . . . . . . . . . 2
2.1 How Many TOP Genes? ....... . . . . . . . . . . . . . . 2
2.2 How Much TOP mRNA? .................... 3
3 Phenomenology of the Translational Regulation
of TOP mRNAs . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 General and Specific Regulation of Protein Synthesis . 4
3.2 Translational Regulation of TOP mRNAs ........ 4
4 Some Features of TOP mRNA
and of Its Translational Regulation ............. 6
4.1 TOP mRNAs Are Capped. . . . . . . . . . . . . . . . . . . . 6
4.2 TOP mRNAs Extracted from Inactive mRNPs
Are Translatable . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3 Bimodal Distribution of TOP mRNA . . . . . . . . . . . . 7
4.4 Quantitative Differences in Polysome Association
of Different TOP mRNAs . . . . . . . . . . . . . . . . . . . . 7
4.5 Relocation of TOP mRNA Between Polysomes
and mRNPs Is Reversible and Fast ............. 8
5 Mechanism of Translational Regulation
of TOP mRNAs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1 Translational Regulation of TOP mRNA
Does Not Involve a Feedback Inhibition
by the Translation Product ................... 9
5.2 Effect of the Amount of Ribosomes
on TOP mRNA Translation. . . . . . . . . . . . . . . . . . . 9
5.3 The 5'UTR Is the cis-Acting Element
of the Translational Control. . . . . . . . . . . . . . . . . . . 10
5.4 Some Proteins Interact with the 5'UTR
of TOP mRNAs . . . . . . . . . . . . . . . . . . . . . . . . . .. 11
5.5 Translational Regulation of TOP mRNA
Is Not Due to Limiting Amounts of eIF-4E ....... 11
x Contents
5.6 Effect of the Phosphorylation State
of Ribosomal Protein S6 on Translational Regulation
of TOP mRNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.7 Global vs. TOP mRNA Translational Regulation. . . . 13
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
RNase L: Effector Nuclease of an Activatable
RNA Degradation System in Mammals. . . . . . . . . . . . . . . . 19
C. BISBAL
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2 The Interferons. . . . . . . . . . . . . . . . . . . . . . . . . . .. 20
3 The 2-5A Pathway ......................... 20
3.1 The 2-5A Synthetases ....................... 21
3.2 2-5A................................... 24
4 RNase L ................................ 24
4.1 Detection of RNase L . . . . . . . . . . . . . . . . . . . . . .. 25
4.2 Subcellular Localization of RNase L ............ 25
4.3 Structure of RNase L ....................... 26
4.4 RNase L Activity .......................... 27
5 RLI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 30
3' Untranslated Regions of c-myc and c-fos mRNAs:
Multifunctional Elements Regulating mRNA Translation,
Degradation and Subcellular Localization . . . . . . . . . . . . .. 35
J.L. VEYRUNE, J. HESKETH and J.M. BLANCHARD
c-fos and c-myc mRNA Degradation and Translation. 35
1.1 3'UTR and mRNA Decay . . . . . . . . . . . . . . . . . . .. 35
1.2 3'UTR and mRNA Translation . . . . . . . . . . . . . . .. 38
1.3 Is There a Link Between Translation
and 3'UTR-Directed Degradation? . . . . . . . . . . . . .. 39
2 Localization of mRNAs in the Cytoplasm
and Their Association with the Cytoskeleton . . . . . .. 42
2.1 Localization of mRNAs ..................... 43
2.2 Association of mRNAs and Polysomes
with the Cytoskeleton: Cytoskeletal-Bound Polysomes 45
2.3 Cytoskeletal-Bound Polysomes Are Enriched
in Specific mRNAs Including c-myc and c-fos . . . . .. 48
2.4 Targeting of c-myc to the Cytoskeletal
and the Perinuclear Cytoplasm: Role of the 3'UTR .. 49
2.5 Localization Signals in c-myc mRNA . . . . . . . . . . .. 53
3 Summary and Future Perspectives .............. 56
References ................................... 56
