Tapasin

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Tapasin

by Christopher Lee

Introduction Structure Functions Potential Binding Viruses Sources

Introduction Top Structure Functions Potential Binding Viruses Sources
Tapasin is a type I membrane glycoprotein which is part of the transport associated with antigen processing (TAP). TAP is a heterodimer complex on the membrane of the endoplasmic reticulum, whose function is to translocate peptide fragments of around 15 amino acids into the lumen of the ER, for loading of the major histocompatability complex (MHC) class I. The loaded MHC I molecule is transported to the cell surface, where cytotoxic CD8⁺ cells trigger a response to unrecognized peptide fragments, presumably from a virus or other foreign entity in the cell. TAP is an adenosine triphosphate (ATP) binding cassette transporter (ABC), and is comprised chiefly of two subunits, TAP1 and TAP2, which are encoded in the vast MHC gene complex. Along with TAP1/2 are calnexin, calreticulin, and tapasin proteins, which are associated with the bridging of the TAP complex with the MHC I for peptide loading. Their roles are summarized in Figure 1. For more information, see Dr. A. Malcolm Campbell's animation of MHC I peptide loading.

Figure 1: The process of transporting peptide fragments from the cytosol into the ER lumen. Proteins are degraded by a proteosome, and the resulting fragments are transported by the TAP complex (TAP1/2) into the lumen. Tapasin mediates this process, and allows imported peptides to bind to MHC I. Calreticulin is also associated with the MHC I molecule, and calnexin stabilizes the MHC I a before the b-2 protein associates to form the MHC I dimer, but calnexin does not play a role once the MHC I is assembled. Once a peptide is bound by MHC I, the MHC I complex is sent to the cell surface via the Golgi complex, to await possible recognition by a cytotoxic CD8⁺ T-cell. Figure drawn by author.

Structure Top Introduction Functions Potential Binding Viruses Sources
Tapasin is a 48 kDa glycoprotein with 448 amino acids and a single N-linked glycosolation site (Li et al., 1999). There is also a transmembrane sequence of 15 hydrophobic amino acids near the carboxy terminus (Ortmann et al., 1997). While the TAP subunits are MHC-encoded, the gene for tapasin is located on the short arm of chromosome 6. Figure 2 provides the entire sequence.

Figure 2: The compared sequences of human and mouse tapasin proteins. 78% of the residues are identical. The solidly underlined amino acids represent the N-terminal signal sequence, and the dotted underlined sequence represent the C-terminal signal sequence. The N-glycosylation site is in bold at aa#253. Figure reproduced with permission from the author, Li et al., 1999.

Tapasin is normally found in stoichiometric amounts with TAP1/2 (Ortmann et al., 1997), which is expected under the predominate hypothesis that tapasin forms a complex with TAP1/2. Tapasin associates with the TAP1 protein of the TAP complex, and appears to bridge TAP and MHC I. (Humans MHC genes are also referred to as HLA, or human leukocite antigen.) TAP1, TAP2, and tapasin form a trimer, with calreticulin and MHC I more transiently associated (Li et al., 1999). It seems likely that TAP2 binds first with TAP1 to form a dimer, then the dimer associates with tapasin, which then associates with MHC I-calreticulin complex (Li et al., 1999) This sequence seems to imply that tapasin enjoys a broad role in the overall process of importing peptide fragments and loading them into MHC I molecules.

Functions Top Introduction Structure Potential Binding Viruses Sources

Besides physically joining MHC I and TAP complexes, tapasin appears to play a role in peptide loading of empty MHC molecules. There has also been evidence linking tapasin to efficient binding of peptide fragments to TAP. The presence of tapasin increases the concentration of TAP complexes in the ER (Lehner et al., 1999), magnifying the amount of peptides which can be introduced into the ER. The chaperone molecule of MHC I, calreticulin, associates only when the MHC I is empty, but the TAP1/2-tapasin trimer releases MHC I molecules when peptides are bound (Li et al., 1999). It is hypothesized that this mechanism allows tapasin to regulate MHC I delivery to the cell surface by retaining empty MHC I molecules in the ER until a peptide can bind.

Many of tapasin's functions has been discovered through the 721.220 human mutant line, in which MHC I complexes are not displayed on the surface of the protein, and tapasin is not expressed (Ortmann et al., 1997). TAP1/2 are normally expressed, and peptide fragments are translocated into the lumen of the ER (Li et al., 1999). The malfunction lies in the inability of MHC I to associate with the TAP complex. In fact, the MHC I dimer appears to lack any associated proteins (Li, et al., 1999). Transfection of tapasin into 721.220 cells results in revitalization of MHC I-TAP complexes, and MHC I-peptide bonding. In two MHC alleles (HLA.A1 and HLA.B8), this tapasin transfection increased surface MHC I presence ten-fold (Ortmann et al., 1997). Other alleles also show increased MHC expression (Lavau et al., 1999). This strongly implicates tapasin as the bridge between TAP and MHC I, and similar results in a study of b2-m-deficient Daudi cells further indicate that tapasin binds to the MHC I-alpha, or -heavy, chain (Li et al., 1997).

A soluble version of tapasin (without the membranal region) associates with MHC I but not with TAP. The transfection of soluble tapasin into the .220 cell line results in restored surface expression of MHC I molecules (Lehner et al., 1999), but peptide translocation remains deficient. This is explained by the suspected function of tapasin to regulate release of MHC I from the ER to the Golgi complex (Schoenhals et al., 1999). When membranal tapasin is not expressed, this regulation is lost, and MHC is released to the surface more readily (Li et al., 2000).

Potential Binding Viruses Top Introduction Structure Functions Sources

There are a number of viruses which inhibit TAP transport of peptides by binding to the TAP complex, preventing peptide translocation from the cytosol to the ER, thereby preventing the host from recognizing viral activity in the cell. The herpes simplex virus is one such virus, but experiments involving normal HLA alleles, and the 721.220 mutant result in virtually idential inhibition of TAP transport, indicating that tapasin is not directly attacked by the virus (Lacaille and Androlewicz, 1998). The human cytomegalovirus US6 gene encodes a 22 kDa protein which also binds the TAP complex, and there has been some speculation that this protein could bind to one of the TAP subunits, or possibly to tapasin (Lehner et al., 1997).

Sources: Top Introduction Structure Functions Potential Binding Viruses

Grandea III AG, Androlewicz MJ, Athwal RS, Geraghty DE, Spies T. 1995 October. Dependence of peptide binding by MHC class I molecules on their interaction with TAP. Science. 270: 105-108.

Lacaille VG, Androlewicz MJ. 1998 July 10. Herpes simplex virus inhibitor ICP47 destabilizes the transporter associated with antigen processing (TAP) heterodimer. Journal of Biological Chemistry. 273: 17386-17390. Abstract and Full Text

Lauvau G, Gubler B, Cohen H, Daniel S, Caillat-Zucman S, van Endert PM. 1999 October. Tapasin enhances assembly of transporters associated with antigen processing-dependednt and -independent peptides with HLA-A2 and HLA-B27 expressed in insect cells. The Journal of Biological Chemistry. 274:31349-58. Abstract and Full Text

Lehner PJ, Surman MJ, Cresswell P. 1998 Feb. Soluble tapasin restores MHC class I expression and function in the tapasin-negative cell line .220. Immunity. 8(2):221-31 Medline Abstract

Lehner PJ, Karttunen JT, Wilkinson GWG, Cresswell P. 1997 June. The human cytomegalovirus US6 glycoprotein inhibits transporter associated with antigen processing-dependent peptide translocation. Proceedings of the National Acadademy of Science. 94: 6904-6909. Abstract and Full Text

Li S, Paulsson KM, Sjögren H, Wang P. 1999 March. Peptide-bound major histocompatibility complex class I molecules associate with tapasin before dissociation from transporter associated with antigen processing. The Journal of Biological Chemistry. 274:8649-54. Abstract and Full Text

Li S, Paulsson KM, Chen S, Sjögren H, Wang P. 2000 January. Tapasin is required for efficient peptide binding to transporter associated with antigen proceesing. The Journal of Biological Chemistry. 275:1581-86. Abstract Full Text (requires subscription or article purchase)

Li S, Sjögren H, Hellman U, Pettersson RF, Wang P. 1997 August. Cloning and functional characterization of a subunit of the transporter associated with antigen processing. Proceedings of the National Acadademy of Science. 94: 8708-8713. Full Text

Ortmann B, Copeman J, Lehner PJ, Sadasivan B, Herberg JA, Grandea AG, Riddell SR, Tampe R, Spies T, Trowsdale J, Cresswell P. 1997 August. A critical role for tapasin in the assembly and function of multimerica MHC class I-TAP complexes. Science. 277:1306-9.

Schoenhals GJ, Krishna RM, Grandea III AG, Spies T, Peterson PA, Yang Y, Früh K. 1999. Retention of empty MHC class I molecules by tapasin is essential to reconstitute antigen presentation in invertebrate cells. The European Molecular Biology Organization Journal. 18:743-753. Abstract

Relevant link:
Peter Cresswell, Yale University

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