This document summarizes the process of nuclear export of messenger RNA (mRNA). It begins with an introduction describing how mRNA must be exported from the nucleus to the cytoplasm to be translated into protein. It then discusses the importance of nuclear export and describes the nuclear pore complex that facilitates transport. The document outlines the roles of Ran GTPase and transport receptors in nuclear export. It provides details on the adaptor-receptor system and multistep process of mRNA export, including recruitment of export factors, translocation through the nuclear pore, and release into the cytoplasm. The summary concludes with sections on regulation and quality control of mRNA export.

1. S U BM ITTED TO- S U BM ITTED BY-
D R . J YOTI S AR D AN A AD I TI BAGD I
AS S IS TAN T P R OFES S OR AT M S BT 1 ST S EM .
LACHOO M EM OR IAL COLLEGE OF BIOTECHN O L OG Y .
S CIEN CE AN D TECHN OL O G Y.
LACHOO MEMORIAL COLLEGE OF
SCIENCE AND TECHNOLOGY

2. TOPIC-
NUCLEAR EXPORT OF MESSENGERRNA

3. CONTENTS-
▪ Introduction.
▪ Importance of nuclear export of mRNA.
▪ Nuclearpore complex.
▪ Generalfeatures of the GTPase-Ranand transport receptors.
▪ NuclearExport of RNAs.
▪ Adaptor-ReceptorSystem.
▪ Stepsof Mrna export.
▪ Processof mRNA export.
▪ mRNA export regulation and quality control.
▪ Conclusionsand Perspectives.
▪ References.

4. INTRODUCTION-
▪ Once the processing of an mRNA is completed in the nucleus, it remains
associated with the specific hnRNP proteins in a messenger ribonuclear protein
complex or mRNP.
▪ Before it can be translated into the encoded protein, it must be exported out of the
nucleus into the cytoplasm.
▪ The nucleus is separated from the cytoplasm by two membranes, which form the
nuclear envelope.
▪ like the plasma membrane surrounding the cells, each nuclear membrane
consists of a water-impermeable phospholipid bilayer and various associated
proteins.
▪ Transport of macromolecules including mRNPs, tRNAs, and ribosomal subunits
out of the nucleus and transport of all nuclear proteins translated in the cytoplasm
into the nucleus occur through nuclear pores.

6. IMPORTANCE OF NUCLEAR EXPORT OF mRNA-
▪ Eukaryotic cell consist of various organelles that execute different activities to
sustain a range of cellular functions. The largest among them is the cell nucleus,
which is surrounded by the nuclear envelope (NE) and stores genetic information in
the form of chromatin.
▪ Transcription of genes, processing of various RNAs and replication and repair of
DNA occur in the nucleus, whereas translation of proteins exclusively takes place
on the ribosomes in the cytoplasm.
▪ Due to this physical separation, mRNAs must be exported to the cytoplasm where
they direct protein synthesis, whereas proteins participate in the nuclear activities
are imorted into nucleus. In addition, some type of RNAs reenter to the nucleus
after being exported into the cytoplasm.
▪ Therefore, nucleo-cytoplasmic transport of RNAs and proteins is essential for
eukaryotic gene expression.

7. NUCLEAR PORECOMPLEX-
 The nuclear envelope contains pores used for:
➢ importing proteins into the nucleus
➢ exporting RNAs and proteins from the nucleus
 NPCs are symmetrical structures that are found at sites where the inner and outer
nuclear membrane are fused.
 NPC ranges from 50 MDa in Yeast to 125 MDa in mammals.
 The proteins of NPCs are callednucleoporins,or Nups.
 Each NPC in human cells has a mass of ~120-106 Daltons (40 times that of a
ribosome).
 These huge complexes are composed of more then 30 different nucleoporins or
nups, each present in 8-56 copies per pore consistent with its octagonal symmetry.
 NPCs contain:
➢ fibrils that extend into the cytoplasm
➢ a basket-like structure that extends into the nucleus

8. ▪ The central part or core of the NPC, which contains the channel, appears as
cylindrical structure inserted in nuclear envelope and is composed of 8 spoke ring
complexes sandwiched between nuclear and cytoplasmic rings.
▪ Many nucleoporins contain repeats of short sequences, which are thought to interact
with transport factors i.e., exportins and importins during transport.( FG-
nucleoporins)
Such as: Gly-Leu-Phe-Gly
X-Phe-X-Phe-Gly
X-X-Phe-Gly
▪ All of the nucleoporins of yeast NPCs have been identified. Yeast contain 11 FG-
nucleoporins count for about half the NPC mass.
▪ Short stretches of amino acids rich in leucine act as the most common nuclear
export sequences.
▪ Translocation of most proteins and ribonucleoprotein complexes through the NPC is
a process that involves specific signals recognised by specific receptors.
▪ A nuclear export receptor:
• binds proteins that contain nuclear export sequences (NESs) in the nucleus
• transports them to the cytoplasm.

11. General features of the GTPase Ran and transport receptors-
▪ Translocation of most proteins and RNP complexes through the nuclear pores is a
process that involves specific signals recognised by specific receptors.
▪ With the exception of mRNA, these targeting signals are recognised by the
family of receptors named as importins and exportins that are responsible for the
majority of nucleo-cytoplasmic exchanges in eukaryotes.
▪ Ran is an abundant cellular GTPase and member of the RAS superfamily. Like
other GTPases, ran exists in either a GDP or a GTP bound state and uses
regulatory proteins to stimulate inter-conversion between two guanine nucleotide
forms.
▪ The Ran GTPase activating protein is located on the cytoplasmic face of the
NPC, as well as in the cytoplasm.
▪ RanGTP binding to the export complexes, but GTP hydrolysis on Ran is not
required for their translocation.

14. NUCLEAR EXPORT OF RNAs-
▪ Transport is an activeprocess and specific,not all RNAs are transported
out, only those specifiedand markedare transported.
▪ None of the cytoplasmicRNAs that are transported into cytoplasmreturn
to the nucleuswith certainexceptionssuch as few snRNAs.
▪ Transportation of ribosomes, tRNA, snRNA and other NC RNAs and
mRNA is highly regulated.
▪ The mRNA is transportedin the form of ribonucleic protein.
▪ Some RNAs such as tRNA, micro RNA, Adeno viral RNA are adaptor
independentinsteadthese are exportin dependentnuclearexport of
mRNAs.
▪ Other RNAs as rRNA, snRNA, viral RNAs, mRNAs require adaptor
proteins along with specific receptorsfor the exportation onto the
cytoplasmthrough NPC.

16. ADAPTOR RECEPTOR SYSTEM-
▪ A considerable number of factors, including numerous mRNA binding proteins,
nucleoporins and ATPase/RNA helicases have been implicated in the export of
mRNA.
▪ Studies in yeast and metazoans have revealed several highly conserved proteins
known as nuclear export factors (NXF) which are specifically required for mRNA
export.
▪ The best characterised factor of NXF family is the S. cerevisisae protein Mex67p.
▪ Mex67p do not bind the GTPase Ran but present the main features of mRNA export
receptors, as they shuttle between nucleus and cytoplasm, interact with FG-
nucleoporins and cross link to poly(A)+ RNA in vivo.
▪ The central domain of Mex67p heterodimerizes with Mtr2p respectively.
▪ Mtr2p is required for optimal binding of Mex67p to NPC, by facilitating the
interaction of a hydrophobic pocket in the domain with FG-repeats.

17. ▪ Mex67p exhibit low affinity for RNA, their interaction with the mRNAs is likely
to be mediated by adaptor proteins.
▪ The most conspicuous adaptor is the essential hn-RNA like protein Yra1p in
yeast. Yra1p belongs to REF family of proteins.
▪ Yra1p/REF is required for mRNA export in yeast and facilitates the recuritment
of Mex67p to mRNP.
▪ Several shuttling hnRNP like proteins in yeast, including Np13p, Nab2p, Hrb1p,
or Hrp1p are essential for yeast mRNA export.
▪ This co-ordinated system is known as adaptor receptor system.

19. STEPS OF mRNA EXPORT-
▪ Processing of pre mRNA and packaging into mRNP complex.
▪ Surveillance of mRNP.
▪ Translocation of mRNP complex through nuclear pore.
▪ mRNPs are directionally released into the cytoplasm for translation.
▪ Recycling .

21. PROCESS OFmRNAEXPORT-
▪ mRNA export requires that newly synthesised precursor mRNAs undergo
several processing steps, which includes 5’ capping and, splicing, 3’ end cleavage
and polyadenylation.
▪ A considerable number of factors, including numerous mRNA binding proteins,
nucleoporins and ATPase/RNA helicases have been implicated in the export of
mRNA.
▪ Once recruited t the fully mature mRNP, the Mex67p-Mtr2p heterodimers
promote the export of the whole complex to the cytoplasm through direct
interactions with FG-nucleoporins lining the NPC channel.
▪ He yeast Np15 were proposed to act in a terminal step of mRNA export by a
binding site for Dbp5p, an ATPase/RNA helicase essential for mRNA export.
▪ Dbp5p is a shuttling protein that associates with mRNP during export.

22. ▪ It is proposed that strong association of Dbp5p with the cytoplasmic of the NPC
at a last stage of translocation contributes to mRNA export directionally by
triggering the remodelling and release of mRNP particles into the cytoplasm.
▪ The mRNPs dissociated in the cytoplasm are transported back to the nucleus by
the means of several pathways.
▪ This cycle continues for the export of mRNAs from nucleus to cytoplasm.

24. Step 1 Step 2
Step 3 Step 4

26. mRNA EXPORT REGULATION AND QUALITY CONTROL-
▪ Diverse mechanisms regulate the nucleo-cytoplasmic transport of proteins and
RNAs.
▪ mRNA export appears to be regulated by phosphorylation. SR- like protein
Nup13p as an additional adaptor for the export receptor Mex67p.
▪ Nup13p is recruited to nascent mRNPs in its phosphorylated form, but interacts
with Mex67p only in its unphosphorylated form.
▪ Glc7p, a phosphatase essential for mRNA export, coordinates dephosphorylation
of Nup13p with the release of mRNP from the 3’ end processing machinery and
the recruitment of Mex67p to the mRNP. Such a mechanism may ensure that
only correctly 3’ end processed mRNPs become associated with the export
receptor.

27. ▪ Regulation of mRNA export also occurs under stress, when heat-induced
transcripts are efficiently exported but non-heat shock mRNAs exhibit a reduced
ability to reach the cytoplasm.
▪ These observations indicate the existence of checkpoint mechanisms, which
ensure that only correctly processed transcripts are released from their site of
synthesis and exported to the cytoplasm. The defect of transcript release may be
indirect and due to the inability of these mutants to terminate export and recycle
factors required for proper 3’ end formation.

28. CONCLUSIONS AND PERSPECTIVES-
▪ Extensive studies have clarified the molecular mechanisms of mRNA export.
Nuclear mRNA export is fully integrated into gene expression, and it proceeds
with other elementary steps of gene expression.
▪ The mRNA exporters plays a crucial role in the coupling of these processes
through the extensive interaction networks with the factors involved in
transcription, polyadenylation, and nuclear export.
▪ While nuclear export is essential for eukaryotic cells, it is also crucial for certain
pathogens, such as viruses that replicate in the host cell nucleus. As studies have
exemplified, the transport receptor tap-p 15 and the TREX components are
exploited to transport viral mRNAs.

29. ▪ Although the details remain enigmatic, the mRNA export pathway may
include various sub routes that are differently dependent on particular adaptor
proteins.
▪ Therefore, a more detailed dissection of the nuclear mRNA export pathway
in mammalian cells will be beneficial not only to better understand the general
gene expression mechanism, but also provide information for more practical
research applications, such as the development of anti-viral drugs.

31. REFERENCES-
▪ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4488659/
▪ Nuclear export of mRNA, Daniel Zenklusen, Franc°oise Stutz* Institute
of Microbiology, Centre Hospitalier Universitaire Vaudois, 44, rue du
Bugnon, 1011 Lausanne, Switzerland Received 4 May 2001; accepted 4
May 2001 First published online 17 May 2001
▪ Nuclear export of RNA, Manuel S. Rodriguez a, Catherine Dargemont
a, Françoise Stutz b,* a Institut Jacques Monod, UMR 7592, CNRS,
Universités Paris VI and VII, 2 Place Jussieu, Tour 43, 75251 Paris
Cedex 05, France. b Dept. of Cell Biology, Sciences III, 30 Quai E.
Ansermet, 1211 Geneva, 4. Switzerland. Received 12 February 2004;
accepted 19 April 2004.

32. Thank you!