Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE TRANSPORT OF MOLECULES BETWEEN THE NUCLEUS AND THE CYTOSOL
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Summary
Eukaryotic cells contain intracellular membrane-enclosed organelles that make up
nearly half the cell’s total volume. The main ones present in all eukaryotic cells are
the endoplasmic reticulum, Golgi apparatus, nucleus, mitochondria, lysosomes,
endosomes, and peroxisomes; plant cells also contain plastids such as chloroplasts.
These organelles contain distinct sets of proteins, which mediate each organelle’s
unique function.
Each newly synthesized organelle protein must find its way from a ribosome in
the cytosol, where the protein is made, to the organelle where it functions. It does
so by following a specific pathway, guided by sorting signals in its amino acid
sequence that function as either signal sequences or signal patches. Sorting signals
are recognized by complementary sorting receptors, which deliver the protein to the
appropriate target organelle. Proteins that function in the cytosol do not contain
sorting signals and therefore remain there after they are synthesized.
During cell division, organelles such as the ER and mitochondria are distributed
to each daughter cell. These organelles contain information that is required for their
construction, and so they cannot be made de novo.
The Transport of Molecules Between the
Nucleus and the Cytosol
The nuclear envelope encloses the DNA and defines the nuclear compartment.
This envelope consists of two concentric membranes, which are penetrated by
nuclear pore complexes (Figure 12–7). Although the inner and outer nuclear
membranes are continuous, they maintain distinct protein compositions. The
inner nuclear membrane contains proteins that act as binding sites for chromosomes
and for the nuclear lamina, a protein meshwork that provides structural
support for the nuclear envelope; the lamina also acts as an anchoring site for
chromosomes and the cytoplasmic cytoskeleton (via protein complexes that span
the nuclear envelope). The inner membrane is surrounded by the outer nuclear
membrane, which is continuous with the membrane of the ER. Like the ER membrane
(discussed later), the outer nuclear membrane is studded with ribosomes
engaged in protein synthesis. The proteins made on these ribosomes are transported
into the space between the inner and outer nuclear membranes (the perinuclear
space), which is continuous with the ER lumen (see Figure 12–7).
Bidirectional traffic occurs continuously between the cytosol and the nucleus.
The many proteins that function in the nucleus—including histones, DNA polymerases,
RNA polymerases, transcriptional regulators, and RNA-processing proteins—are
selectively imported into the nuclear compartment from the cytosol,
where they are made. At the same time, almost all RNAs—including mRNAs,
rRNAs, tRNAs, miRNAs, and snRNAs—are synthesized in the nuclear compartment
and then exported to the cytosol. Like the import process, the export process
is selective; mRNAs, for example, are exported only after they have been properly
modified by RNA-processing reactions in the nucleus. In some cases, the transport
process is complex. Ribosomal proteins, for instance, are made in the cytosol
and imported into the nucleus, where they assemble with newly made ribosomal
RNA into particles. The particles are then exported to the cytosol, where they
assemble into ribosomes. Each of these steps requires selective transport across
the nuclear envelope.
NUCLEUS
PLASTIDS
MITOCHONDRIA
PEROXISOMES
ENDOPLASMIC RETICULUM
LATE ENDOSOME
LYSOSOME
EARLY ENDOSOME
CYTOSOL
GOLGI
CELL EXTERIOR
SECRETORY
VESICLES
MBoC6 mp669/p704
Nuclear Pore Complexes Perforate the Nuclear Envelope
Large and elaborate nuclear pore complexes (NPCs) perforate the nuclear envelope
in all eukaryotes. Each NPC is composed of a set of approximately 30 different
proteins, or nucleoporins. Reflecting the high degree of internal symmetry,
each nucleoporin is present in multiple copies, resulting in 500–1000 protein molecules
in the fully assembled NPC, with an estimated mass of 66 million daltons in
yeast and 125 million daltons in vertebrates (Figure 12–8). Most nucleoporins are