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which this occurs is under investigation ( Grunwald et al. 2011, Kriwacki and
Yoon 2011 ). Approximately 400 polypeptide chains (nucleoporins) form the
cylindrical nuclear pore ( Kriwacki and Yoon 2011 ).
mRNA is transported from the nucleus to the cytoplasm through the nuclear
pore after synthesis and processing within the nucleus. Proteins required for
nuclear functions (such as transcription factors, histones, DNA polymerases, RNA
polymerases, and splicing factors) are manufactured in the cytoplasm and trans-
ported into the nucleus. Transport of molecules through the pore can be passive
or an active, energy-dependent process. The passive transfer of molecules through
the nuclear pore complex involves only small molecules and proteins < 50 kDa.
Most proteins and RNAs are transported actively into and out of the nucleus.
During active transport, the nuclear pore can open to a diameter of > 25 nm.
Some proteins are recognized and transported because they carry specific sig-
nals (exportins) that bind to a receptor on the transport machinery ( Nakielny
and Dreyfuss 1999 ).
Pre-mRNAs and mRNAs are associated with a set of at least 20 proteins, form-
ing a heterogeneous nuclear ribonucleoprotein complex (hnRNP) throughout
their processing in the nucleus and transport to the cytoplasm. At least two of
the hnRNP proteins contain nuclear export signals.
snRNAs function within the nucleus as components of the RNA processing
machinery. These RNAs are initially transported from the nucleus to the cyto-
plasm where they associate with proteins to form functional small nuclear ribo-
nucleoproteins, and then they return to the nucleus. Proteins that bind to the
5 caps of snRNAs seem to be involved in the export of the snRNAs to the cyto-
plasm, whereas other sequences are responsible for their transport from the
cytoplasm into the nucleus ( Cooper 2000 ).
2.11 Transport of Proteins within the Cytoplasm
Proteins are transferred into the endoplasmic reticulum when they are trans-
lated on membrane-bound ribosomes. The endoplasmic reticulum (ER) is a net-
work of membrane-enclosed tubules and sacs (cisternae) that extends from
the nuclear membrane throughout the cytoplasm. The entire ER is enclosed by
a continuous membrane and is the largest organelle of most eukaryotic cells.
Proteins destined to remain in the cytosol (the fluid portion of the cytoplasm,
excluding organelles) or to be incorporated into the nucleus or mitochondria
are synthesized on free ribosomes and released into the cytosol when their
translation is complete.
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