TAP

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TAP

• Transporter associated with antigen processing.
• ABC Subfamily B.

http//www.bio.davidson.edu/courses/Immunology/Stu
dents/spring2000/buxton/MFIP.htm
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• 2 units TAP1 and TAP2 heterodimer.
• 10 TM helices 9 TM helices
• Located in ER membrane due to signals in the
  transmembrane region.

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Mechanism
1. Peptide binds
Cytosol
ER
Fast! High Affinity
Slow Conformational Change
2. ATP binds hydrolyses
2nd Conformational Change Translocation of
peptide Lateral mobility
ATP
ADP
Cytosol
Pi
ER
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Type 1 MHCs

• Found on all body cells.
• Present endogenous peptides to
• immune system.
• Bind to cytotoxic CD8 T-cells.
• Assembled in the ER, to form large macromolecular
  loading complex which includes TAP.
• Cytotoxic T-cells not activated in non infected
  state due to elimination during development in
  the thymus.

http//library.thinkquest.org/03oct/01254/binding.
htm Courtesy of Biology, 5th editionCampbell,
Reese, and Mitchell
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Macromolecular loading complex

• An essential part of TAP function, for loading
  transported peptides onto MHC1.
• Composed of one TAP which interacts with four
  MHC1s via a protein called tapasin.
• Tapasin is involved in
• - recruiting other components of the complex.
• - mediating efficient interaction between TAP
  and MHC1.
• - stabilising TAP and increases its half-life.
• ? increased peptide transport rate.

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Nature Immunology  5, 678 - 684 (2004) Published
online 28 June 2004
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Macromolecular loading complex

• Chaperone molecules oversee assembly of the
  macromolecular complex
• - Calnexin ensures correct protein folding of
  MHC1 a-chains.
• - Calreticulin later replaces calnexin and
  continues monitoring folding.
• - Thiol reductase ERp57 oversees formation of
  disulphide bridges.
• On successful transferral of the peptide, the MHC
  dissociates from TAP.

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Viruses

• Viruses can interfere with antigen presentation
  to avoid immune surveillance ? chronic/latent
  infection.
• This can occur by disturbing
• 1. the generation of peptides.
• 2. the export of class I MHCs to cell surface.
• 3. macromolecular loading complex assembly.
• 4. the transport of peptides by TAP. This can
  be done by decreasing TAP expression or
  blocking transport directly.

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Herpes Simplex Virus 1 and 2

• Codes for the immediate early protein ICP47.
• Binds with high affinity to the cytosolic side of
  TAP.
• It both competes for binding and may induce a
  conformational change in TAP.
• It is species specific as it displays a 100 times
  higher affinity for human TAP than murine TAP.

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Tumours

• Tumour cells with diminished TAP levels
  translocate less peptide for MHC presentation,
  allowing them to escape detection by the immune
  system.
• Decrease in TAP levels
• - Secretion of cytokine interleukin-10 by
  tumour cells
• - Mutations in TAP gene (chromosome 6, band
  p21.3)
• - Mostly associated with low TAP mRNA levels
  (either low transcription, or mRNA degradation)

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Tumours- a cause?

• Studies suggest TAP down regulation is linked to
  increased disease progression low TAP levels in
  melanoma confer a reduced chance of survival.
• Conflicting evidence finds TAP expression not to
  be significantly associated with tumour grading
  and staging system.
• TAP -ve cells are connected to large persistent
  tumours whilst TAP ve connected to short lived
  tumours.
• Decreased TAP is seen more commonly in metastatic
  tumours than primary tumours.

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• Turning on the immune system against tumours-
• A possible cure?

http//www.research.vt.edu/resmag/sciencecol/cance
r97.html
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Tumours- a cure?

• Interferon-? can restore TAP expression in cases
  of low TAP mRNA levels increases promoter
  activity.
• Mice given exogenous TAP showed
• - a clear regression in tumour size as
  recognition by the cytotoxic T-cells
  occurred.
• - protective immunity when presented with
  cancer cells from the same cell line.

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Some Examples

• TAP and Small Cell Lung Cancer
• -A single amino acid exchange at position 659
  of TAP 1 between C motif and Walker B motif.
• -This interferes with ATP binding or hydrolysis
  rendering the TAP complex unfunctional.
• Human Sk-MEL-10 melanoma cell line
• -Increased rate of TAP1 mRNA degradation,
  with a single nucleotide deletion.
• -IFN-? has no effect.

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References

• Abele, R., Tampe, R. (2005) FEBS letters, 580,
  1156-1163
• Lankat-Buttgereit, B., Tampe, R. (2003) in ABC
  Proteins From Bacteria to Man, pp533-550
  (Hollan, I.B., Cole, S.P.C., Kuchler, K. and
  Higgins, C.F., Eds.) Academic Press, New York
• Tomazin, R., et al. (1998) Journal of Virology,
  72, 2560-2563
• Seiliger, B., Maeurer, MJ., Ferrone, S. (2000)
  Immunology Today, 21, (9) 455-464
• Fowler, NL., Frazer, IH. (2004) Gynecologic
  Oncology, 92, 914-921
• Lou, Y., et al. (2005) Cancer Research, 65, (17),
  7926-7933
• Yang, T., McNally, BA., Ferrone, S., Liu, Y.,
  ZHeng, P. (2003) Journal of Biological Chemistry,
  278 (17) 15291-15296

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ANY QUESTIONS??
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Type II MHCs

• Presents exogenous peptides after internalisation
  and degradation.
• Found only on specific cells in the immune
  system.
• Bind to CD4 T-cells.

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TAP and the Immune System.

• Major Histocompatibility Complexes present self
  or foreign peptide fragments on cell surface.
• This allows the immune system to constantly
  monitor for foreign peptides.
• TAP is the ABC protein responsible for the
  transport of these peptides from the cytosol into
  the endoplasmic reticulum (ER).
• Peptide loading occurs in the ER using the
  macromolecular loading complex.

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Human Cytomegalovirus

• GpUS6, a glycoprotein in the ER binds to luminal
  part of the TAP complex.
• Binding of TAP, tapasin, calreticulin, class I
  MHC unaffected.
• Stabilises a form of TAP 1 that cannot bind ATP.
• Can be overridden with TAP over expression using
  interferon-?.