Biology of Disease

16.2 Nucleus and Nucleocytoplasmic Transport

The nucleus contains the genome of the cell stored as sequences of nucle-

otide bases in DNA molecules most of which are contained in chromosomes

(Chapter 15). Other than a few mitochondrial genes (Section 10.4), all the

human genes are found here. Surprisingly, it seems that less than 5% of the

three billion pairs of nucleotides constitute the less than 30 000 genes of

humans. The rest of the DNA is found interspersed as introns (Chapter 13) in

between individual genes and as repetitive sequences. The function of these

short sequences is unknown, although telomeres (Chapter 18) stabilize the

ends of chromosomes and centromeres (Chapter 15) allow the spindle to

attach to the chromosomes during cell division.

All cells, with the exception of mature mammalian erythrocytes, possess a

nucleus that contains the chromosomes. The nucleus is separated from the

cytoplasm by a nuclear envelope consisting of outer and inner nuclear mem-

branes (Figure 16.1). Nuclear pores in the envelope (Figure 16.1 (B) and (C))

allow the transport of proteins into the nucleus and the export of ribosomal

subunits, transfer RNA and messenger RNA molecules to the cytoplasm. This

movement between the nucleus and cytoplasm is called nucleocytoplasmic

transport. Nuclear pores have elaborate structures called nuclear pore com-

plexes (NPCs) that regulate nucleocytoplasmic transport. The best studied

NPCs are those from amphibians, such as the toad Xenopus laevis. Each NPC

consists of around 100 proteins and has a Mr of approximately 125 x 10^6. These

complexes are cylindrical with a ring-like structure of eightfold symmetry con-

taining a central core. Fibrils, 50 to 100 nm long, extend from the cylinder into

the cytoplasm while the inner surface has a basket-like attachment extend-

ing into the nucleoplasm. The 5600 pores of a typical mammalian nucleus

are held in place by attachments to the nuclear envelope and to a scaffold of

fibrous proteins called the lamina that lines and supports the inner face of the

nucleus (Figure 16.1(A)).

Nucleocytoplasmic transport is a rather complicated process and only a sim-

plified view of protein import is given here (Figure 16.2). Generally, proteins

with a Mr larger than 30 000 cannot enter the nucleus by free diffusion but they

can enter by active transport if they have a nuclear locating signal (NLS). This

also requires the participation of a number of soluble import factors. The best

known NLSs are rich in basic amino acid residues, such as that of the large T

antigen of the simian virus 40 which has a sequence: Pro-Lys-Lys-Lys-Arg-Lys-

Val. A protein with a NLS is called a cargo. An adapter protein recognizes the

NLS signal of the cargo and binds to it. This complex then binds to an import

receptor (Figure 16.2). Importin @ and importin A are the best characterized

adapter and import receptors respectively. This tripartite complex then passes

through a NPC into the nucleus where it binds to a small protein called Ran.

Ran is a GTPase, which can bind and then hydrolyze GTP to GDP. The con-

formation of Ran depends upon whether it has a bound GTP or GDP. The

binding of Ran-GTP to the complex releases the cargo-adaptor in the nucleus

and allows the import receptor-Ran-GTP to be shuttled to the cytoplasm. The

adapter now releases its cargo within the nucleoplasm. Within the cytoplasm,

yet another protein, GTPase-accelerating protein (Ran-Gap, not shown in

Figure 16.2 for the sake of clarity) stimulates Ran to hydrolyze its bound GTP.

The conformation of Ran-GDP has only a low affinity for the import receptor,

which is then released and can participate in a new cycle of nuclear import.

The Ran-GDP is returned to the nucleus where it releases its GDP and binds a

fresh GTP under the influence of a specific guanine nucleotide-exchange fac-

tor (Ran-GEF). The adapter in the nucleus binds to a nuclear export receptor

that allows it to be returned to the cytoplasm.

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