TAPTAP

= Transporter associated with antigen = Transporter associated with antigen processing.processing.

ABC Subfamily B.ABC Subfamily B.

http://www.bio.davidson.edu/courses/Immunology/Students/spring2000/buxton/MFIP.htm

2 units – TAP1 and TAP2 = 2 units – TAP1 and TAP2 = heterodimer.heterodimer.

10 TM helices 9 TM helices10 TM helices 9 TM helices Located in ER membrane due to signals in Located in ER membrane due to signals in

the transmembrane region.the transmembrane region.

MechanismMechanism

1. Peptide binds

Fast! High Affinity

Slow Conformational Change

2. ATP binds + hydrolysesATP

Pi

ADP

2nd Conformational Change

Translocation of peptide

Lateral mobility

Cytosol

ER

Cytosol

ER

Type 1 MHCs Type 1 MHCs Found on all body cells.Found on all body cells. Present endogenous peptides to Present endogenous peptides to

immune system.immune system. Bind to cytotoxic CD8+ T-cells.Bind to cytotoxic CD8+ T-cells. Assembled in the ER, to form large Assembled in the ER, to form large

macromolecular loading complex which macromolecular loading complex which includes TAP.includes TAP.

Cytotoxic T-cells not activated in non Cytotoxic T-cells not activated in non infected state due to elimination during infected state due to elimination during development in the thymus.development in the thymus.

http://library.thinkquest.org/03oct/01254/binding.htm

Courtesy of Biology, 5th editionCampbell, Reese, and Mitchell

Macromolecular loading Macromolecular loading complexcomplex

An essential part of TAP function, for loading An essential part of TAP function, for loading transported peptides onto MHC1.transported peptides onto MHC1.

Composed of one TAP which interacts with Composed of one TAP which interacts with four MHC1s via a protein called tapasin. four MHC1s via a protein called tapasin.

Tapasin is involved in: Tapasin is involved in: - recruiting other components of the - recruiting other components of the

complex.complex.- mediating efficient interaction between - mediating efficient interaction between

TAP TAP and MHC1. and MHC1.- stabilising TAP and increases its half-- stabilising TAP and increases its half-

life.life. increased peptide transport rate.increased peptide transport rate.

Nature Immunology  5, 678 - 684 (2004) Published online: 28 June 2004;

MacromolecularMacromolecular loading loading complexcomplex

Chaperone molecules oversee assembly of Chaperone molecules oversee assembly of the macromolecular complex:the macromolecular complex:

- Calnexin ensures correct protein - Calnexin ensures correct protein folding of folding of MHC1 MHC1 αα-chains.-chains.

- Calreticulin later replaces calnexin - Calreticulin later replaces calnexin and and continues monitoring folding. continues monitoring folding.

- Thiol reductase ERp57 oversees - Thiol reductase ERp57 oversees formation of formation of disulphide bridges. disulphide bridges.

On successful transferral of the peptide, On successful transferral of the peptide, the MHC dissociates from TAP. the MHC dissociates from TAP.

VirusesViruses Viruses can interfere with antigen presentation Viruses can interfere with antigen presentation

to avoid immune surveillance to avoid immune surveillance chronic/latent chronic/latent infection.infection.

This can occur by disturbing…This can occur by disturbing…

1. the generation of peptides.1. the generation of peptides.

2. the export of class I MHCs to cell surface.2. the export of class I MHCs to cell surface.

3. macromolecular loading complex assembly.3. macromolecular loading complex assembly.

4. 4. the transport of peptides by TAPthe transport of peptides by TAP. This can . This can be be done by decreasing TAP expression or done by decreasing TAP expression or blocking blocking transport directly. transport directly.

Herpes Simplex Virus 1 Herpes Simplex Virus 1 and 2and 2

Codes for the immediate early protein Codes for the immediate early protein ICP47.ICP47.

Binds with high affinity to the cytosolic Binds with high affinity to the cytosolic side of TAP.side of TAP.

It both competes for binding and may It both competes for binding and may induce a conformational change in TAP.induce a conformational change in TAP.

It is species specific as it displays a 100 It is species specific as it displays a 100 times higher affinity for human TAP times higher affinity for human TAP than murine TAP.than murine TAP.

TumoursTumours Tumour cells with diminished TAP levels Tumour cells with diminished TAP levels

translocate less peptide for MHC translocate less peptide for MHC presentation, allowing them to escape presentation, allowing them to escape detection by the immune system.detection by the immune system.

Decrease in TAP levels:Decrease in TAP levels:

- Secretion of cytokine interleukin-10 - Secretion of cytokine interleukin-10 by tumour by tumour cells cells

- Mutations in TAP gene (chromosome - Mutations in TAP gene (chromosome 6, band 6, band p21.3) p21.3)

- Mostly associated with low TAP - Mostly associated with low TAP mRNA levels mRNA levels (either low transcription, (either low transcription, or mRNA degradation)or mRNA degradation)

Tumours- a cause?Tumours- a cause? Studies suggest TAP down regulation is linked Studies suggest TAP down regulation is linked

to increased disease progression – low TAP to increased disease progression – low TAP levels in melanoma confer a reduced chance of levels in melanoma confer a reduced chance of survival.survival.

Conflicting evidence finds TAP expression not Conflicting evidence finds TAP expression not to be significantly associated with tumour to be significantly associated with tumour grading and staging system. grading and staging system.

TAP -ve cells are connected to large persistent TAP -ve cells are connected to large persistent tumours whilst TAP +ve connected to short tumours whilst TAP +ve connected to short lived tumours.lived tumours.

Decreased TAP is seen more commonly in Decreased TAP is seen more commonly in metastatic tumours than primary tumours.metastatic tumours than primary tumours.

Turning on the immune system against Turning on the immune system against tumours-tumours-

A possible cure?A possible cure?

http://www.research.vt.edu/resmag/sciencecol/cancer97.html

Tumours- a cure?Tumours- a cure?

Interferon-γ 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.

Some ExamplesSome Examples TAP and Small Cell Lung Cancer:TAP and Small Cell Lung Cancer:

-A single amino acid exchange at -A single amino acid exchange at position 659 position 659 of TAP 1 between ‘C’ of TAP 1 between ‘C’ motif and Walker B motif and Walker B motif. motif.

-This interferes with ATP binding or -This interferes with ATP binding or hydrolysis hydrolysis rendering the TAP rendering the TAP complex unfunctional.complex unfunctional.

Human Sk-MEL-10 melanoma cell line:Human Sk-MEL-10 melanoma cell line:-Increased rate of TAP1 mRNA -Increased rate of TAP1 mRNA

degradation, degradation, with a single nucleotide with a single nucleotide deletion.deletion.

-IFN--IFN-γγ has no effect. has no effect.

ReferencesReferences Abele, R., Tampe, R. (2005) FEBS letters, 580, 1156-1163Abele, R., Tampe, R. (2005) FEBS letters, 580, 1156-1163 Lankat-Buttgereit, B., Tampe, R. (2003) in: ABC Proteins: Lankat-Buttgereit, B., Tampe, R. (2003) in: ABC Proteins:

From Bacteria to Man, pp533-550 (Hollan, I.B., Cole, S.P.C., From Bacteria to Man, pp533-550 (Hollan, I.B., Cole, S.P.C., Kuchler, K. and Higgins, C.F., Eds.) Academic Press, New Kuchler, K. and Higgins, C.F., Eds.) Academic Press, New YorkYork

Tomazin, R., et al. (1998) Journal of Virology, 72, 2560-2563Tomazin, R., et al. (1998) Journal of Virology, 72, 2560-2563 Seiliger, B., Maeurer, MJ., Ferrone, S. (2000) Immunology Seiliger, B., Maeurer, MJ., Ferrone, S. (2000) Immunology

Today, 21, (9) 455-464Today, 21, (9) 455-464 Fowler, NL., Frazer, IH. (2004) Gynecologic Oncology, 92, Fowler, NL., Frazer, IH. (2004) Gynecologic Oncology, 92,

914-921914-921 Lou, Y., et al. (2005) Cancer Research, 65, (17), 7926-7933Lou, Y., et al. (2005) Cancer Research, 65, (17), 7926-7933 Yang, T., McNally, BA., Ferrone, S., Liu, Y., ZHeng, P. (2003) Yang, T., McNally, BA., Ferrone, S., Liu, Y., ZHeng, P. (2003)

Journal of Biological Chemistry, 278 (17) 15291-15296Journal of Biological Chemistry, 278 (17) 15291-15296

ANY

QUESTIONS??

Type II MHCsType II MHCs

Presents exogenous peptides after Presents exogenous peptides after internalisation and degradation.internalisation and degradation.

Found only on specific cells in the immune Found only on specific cells in the immune system.system.

Bind to CD4+ T-cells.Bind to CD4+ T-cells.

TAP and the Immune TAP and the Immune System.System.

Major Histocompatibility Complexes Major Histocompatibility Complexes present self or foreign peptide fragments present self or foreign peptide fragments on cell surface.on cell surface.

This allows the immune system to This allows the immune system to constantly monitor for foreign peptides.constantly monitor for foreign peptides.

TAP is the ABC protein responsible for the TAP is the ABC protein responsible for the transport of these peptides from the transport of these peptides from the cytosol into the endoplasmic reticulum cytosol into the endoplasmic reticulum (ER).(ER).

Peptide loading occurs in the ER using the Peptide loading occurs in the ER using the macromolecular loading complex.macromolecular loading complex.

Human CytomegalovirusHuman Cytomegalovirus

GpUS6, a glycoprotein in the ER binds to GpUS6, a glycoprotein in the ER binds to luminal part of the TAP complex.luminal part of the TAP complex.

Binding of TAP, tapasin, calreticulin, class Binding of TAP, tapasin, calreticulin, class I MHC unaffected.I MHC unaffected.

Stabilises a form of TAP 1 that cannot Stabilises a form of TAP 1 that cannot bind ATP.bind ATP.

Can be overridden with TAP over Can be overridden with TAP over expression using interferon-expression using interferon-γγ. .