Nuclear localization sequence

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A nuclear localization signal or sequence (NLS) is an amino acid sequence that 'tags' a protein for import into the cell nucleus by nuclear transport.[1] Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface.[1] Different nuclear localized proteins may share the same NLS.[1] An NLS has the opposite function of a nuclear export signal (NES), which targets proteins out of the nucleus.

Types

Classical

These types of NLSs can be further classified as either monopartite or bipartite. The major structural differences between the two are that the two basic amino acid clusters in bipartite NLSs are separated by a relatively short spacer sequence (hence bipartite - 2 parts), while monopartite NLSs are not. The first NLS to be discovered was the sequence PKKKRKV in the

importin β
. The latter can be considered the actual import mediator.

Chelsky et al. proposed the consensus sequence K-K/R-X-K/R for monopartite NLSs.[3] A Chelsky sequence may, therefore, be part of the downstream basic cluster of a bipartite NLS. Makkah et al. carried out comparative mutagenesis on the nuclear localization signals of SV40 T-Antigen (monopartite), C-myc (monopartite), and nucleoplasmin (bipartite), and showed amino acid features common to all three. The role of neutral and acidic amino acids was shown for the first time in contributing to the efficiency of the NLS.[4]

Rotello et al. compared the nuclear localization efficiencies of eGFP fused NLSs of SV40 Large T-Antigen, nucleoplasmin (AVKRPAATKKAGQAKKKKLD), EGL-13 (MSRRRKANPTKLSENAKKLAKEVEN), c-Myc (PAAKRVKLD) and TUS-protein (KLKIKRPVK) through rapid intracellular protein delivery. They found significantly higher nuclear localization efficiency of c-Myc NLS compared to that of SV40 NLS.[5]

Non-classical

There are many other types of NLS, such as the acidic M9 domain of hnRNP A1, the sequence KIPIK in yeast transcription repressor Matα2, and the complex signals of U snRNPs. Most of these NLSs appear to be recognized directly by specific receptors of the importin β family without the intervention of an importin α-like protein.[6]

A signal that appears to be specific for the massively produced and transported ribosomal proteins,[7][8] seems to come with a specialized set of importin β-like nuclear import receptors.[9]

Recently a class of NLSs known as PY-NLSs has been proposed, originally by Lee et al.

Importin β2 (also known as transportin or karyopherin β2), which then translocates the cargo protein into the nucleus. The structural basis for the binding of the PY-NLS contained in Importin β2 has been determined and an inhibitor of import designed.[11]

Discovery

The presence of the nuclear membrane that sequesters the cellular

eukaryotic cells. The nuclear membrane, therefore, separates the nuclear processes of DNA replication and RNA transcription from the cytoplasmic process of protein production. Proteins required in the nucleus must be directed there by some mechanism. The first direct experimental examination of the ability of nuclear proteins to accumulate in the nucleus was carried out by John Gurdon when he showed that purified nuclear proteins accumulate in the nucleus of frog (Xenopus
) oocytes after being micro-injected into the cytoplasm. These experiments were part of a series that subsequently led to studies of nuclear reprogramming, directly relevant to stem cell research.

The presence of several million pore complexes in the oocyte nuclear membrane and the fact that they appeared to admit many different molecules (insulin, bovine serum albumin, gold nanoparticles) led to the view that the pores are open channels and nuclear proteins freely enter the nucleus through the pore and must accumulate by binding to DNA or some other nuclear component. In other words, there was thought to be no specific transport mechanism.

This view was shown to be incorrect by Dingwall and Laskey in 1982. Using a protein called nucleoplasmin, the archetypal ‘

Ran
.

Mechanism of nuclear import

Proteins gain entry into the nucleus through the nuclear envelope. The nuclear envelope consists of concentric membranes, the outer and the inner membrane. The inner and outer membranes connect at multiple sites, forming channels between the cytoplasm and the nucleoplasm. These channels are occupied by

nuclear pore complexes
(NPCs), complex multiprotein structures that mediate the transport across the nuclear membrane.

A protein translated with an NLS will bind strongly to

Ran-GTP will bind to the importin-protein complex, and its binding will cause the importin to lose affinity for the protein. The protein is released, and now the Ran-GTP/importin complex will move back out of the nucleus through the nuclear pore. A GTPase-activating protein (GAP) in the cytoplasm hydrolyzes the Ran-GTP to GDP, and this causes a conformational change in Ran, ultimately reducing its affinity for importin. Importin is released and Ran-GDP is recycled back to the nucleus where a Guanine nucleotide exchange factor
(GEF) exchanges its GDP back for GTP.

See also

References

  1. ^
    PMID 10494618
    , retrieved 2020-12-17
  2. S2CID 42354737
    .
  3. ^
    PMID 3417784
    .
  4. PMID 8805337
    .
  5. PMID 26011555
    .
  6. PMID 9759490
    .
  7. PMID 10386617
    .
  8. ISBN 978-1-55581-184-6
    .
  9. PMID 9182759
    .
  10. PMID 16901787
    .
  11. PMID 17435768
    .
  12. S2CID 39465973
    .
  13. PMID 1664152
    .
  14. PMID 19001369
    .
  15. PMID 10745017
    .
  16. PMID 9695948
    .
  17. PMID 3417784
    .
  18. S2CID 45889419
    .

Further reading

External links
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