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Transporter associated with antigen processing

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transporter 1, ATP-binding cassette, sub-family B (MDR/TAP)
Identifiers
SymbolTAP1
Alt. symbolsABCB2
NCBI gene6890
HGNC43
OMIM170260
RefSeqNM_000593
UniProtQ03518
Other data
LocusChr. 6 p21.3
Search for
StructuresSwiss-model
DomainsInterPro
transporter 2, ATP-binding cassette, sub-family B (MDR/TAP)
Identifiers
SymbolTAP2
Alt. symbolsABCB3
NCBI gene6891
HGNC44
OMIM170261
RefSeqNM_000544
UniProtQ03519
Other data
LocusChr. 6 p21.3
Search for
StructuresSwiss-model
DomainsInterPro

Transporter associated with antigen processing (TAP) is a member of the ATP-binding-cassette transporter family (ABC transporter).[1] It delivers cytosolic peptides into the endoplasmic reticulum (ER) where they bind to nascent MHC class I molecules. [2]

The TAP structure is formed of two proteins: TAP-1 and TAP-2, each have one hydrophobic region and one ATP-binding region, which assemble into a heterodimer resulting in a four-domain transporter [3]

Function

The TAP transporter is found in the ER lumen associated with the peptide-loading complex (PLC). This complex of β2 microglobulin, calreticulin, ERp57, TAP, tapasin and MHC class I acts to keep hold of MHC molecules until they have been fully loaded with peptides [4]

Peptide Transport

TAP mediated peptide transport is a multistep process and the peptide binding pocket is formed by TAP-1 and TAP-2. Association with TAP is an ATP independent event, ‘in a fast bimolecular association step, peptide binds to TAP, followed by a slow isomerisation of the TAP complex’ [5]. It is suggested that the conformational change in structure triggers ATP hydrolysis and so initiates peptide transport [6] Both nucleotide-binding domains (NBDs) are required for peptide translocation, as each NBD cannot hydrolyse ATP alone. The exact mechanism of transport is not known however findings indicate that ATP binding to TAP-1 is the initial step in the transport process, and that ATP bound to TAP-1 induces ATP binding in TAP-2. It has also been shown that undocking of the loaded MHC class I is linked to the transport cycle of TAP caused by signals from the TAP-1 subunit [7]

Specificity

The ATPase activity of TAP is highly dependant on presence of the correct substrate, and peptide binding is prerequisite for ATP hydrolysis, this prevents waste of ATP via peptide-independent hydrolysis [8]. The specificity of TAP proteins was first investigated by trapping peptides in the ER using glycosylation. TAP binds to peptides with 8-16 residues with equal affinity and translocation is most efficient for peptides 8-12 residues long with efficiency reducing at peptides longer than 12 residues[9]. However peptides with more than 40 residues were translocated albeit with low efficiency. Peptides with low affinity for the MHC class I molecule are transported out of the ER by an efficient ATP-dependant export protein, these outlined mechanisms may represent a mechanism for ensuring only high affinity peptides are bound to MHC class I [10]

See Also

References

  1. ^ Daumke, O. (2001) Functional asymmetry of the ATP-binding-cassettes of the ABC transporter TAP is determined by intrinsic properties of the nucleotide binding domains. Eur. J. Biochem., 268:4776-4786.
  2. ^ Suh, WK. (1994) Interaction of MHC class I molecules with the transporter associated with antigen processing. Science. Vol 264 5163:1322-1326.
  3. ^ Janeway CA, Travers P, Walport M and Shlomchik M. Antigen Presentation to T-lymphocytes. Immunobiology 5th Ed. Ch 5. New York and London: Garland Science 2001
  4. ^ Antoniou AN, Powis SJ, Elliott T. Assembly and export of MHC class I peptide ligands. Current Opinions in Immunology 15(1): 75-81, 200
  5. ^ Van Endert PM, Tampé R, Meyer TH, Tisch R, Bach JF and McDevitt HO. A sequential model for peptide binding and transport by the transporters associated with antigen processing. Immunity 1(6): 491-500, 1994,
  6. ^ Neumann L and Tampé R. Kinetic analysis of peptide binding to the TAP transport complex: evidence for structural rearrangements induced by substrate binding. Journal of molecular biology 294(5): 1203-1213, 1999
  7. ^ Alberts P, Dame O, Everson EV, Howard JC and Kittle MR. Distinct functional properties of the TAP subunits coordinate the nucleotide-dependent transport cycle. Current Biology 11 (4): 242-251, 2001
  8. ^ Neumann L and Tampé R. Kinetic analysis of peptide binding to the TAP transport complex: evidence for structural rearrangements induced by substrate binding. Journal of molecular biology 294(5): 1203-1213, 1999
  9. ^ Neefjes JJ, Momburg F, and Hammerling GJ. Selective and ATP dependent translocation of peptides by the MHC-encoded transporter. Science 261: 769 –771, 199
  10. ^ Lankat-Buttgereit B and Tampé R. The Transporter Associated With Antigen Processing: Function and Implications in Human Diseases. Physiol Rev 82: 187–204, 2002
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