Assembly of major histocompatibility complex (MHC) class I molecules occurs within the endoplasmic reticulum (ER) of cells. Newly synthesized MHC class I molecules are recruited into interactions with the transporter associated with antigen processing (TAP), tapasin, ERp57, protein disulfide isomerase, calnexin and calreticulin. This complex of accessory proteins can be considered a molecular machine whose job it is to (i) pump the peptide products of protein degradation into the region of MHC class I assembly (ii) recruit unassembled MHC class I (iii) facilitate MHC class I-peptide assembly and (iv) ensure regulated release of optimally loaded MHC class I. Much remains to be understood about the workings of this intricate molecular machine, which has been the focus of our research for the past eleven years. Based on our previous work with the TAP transporter, we are able to propose a detailed model for how ATP binding and hydrolysis couple to peptide binding and transport. In the proposed studies we will examine effects of TAP substrates on nucleotide binding and exchange by TAP, and on interactions between the nucleotide binding domains (NBD). A model for the peptide-binding site of TAP will also be examined. These investigations will allow for better understanding of how TAP can be manipulated to enhance or suppress immune responses, and will also allow for better predictions of immunodominant cytotoxic T lymphocyte (CTL) epitopes. Based on analyses of the assembly characteristics of various MHC class I allotypes in tapasin-deficient cells, it is our hypothesis that tapasin is essential for peptide loading of MHC class I allotypes that have slow intrinsic peptide loading kinetics. Peptide binding properties of tapasin dependent and independent MHC class I allotypes will be compared under different conditions. Our data suggest that tapasin is responsible for recruiting calreticulin and ERp57 into the peptide loading complex. Furthermore different conformational states of tapasin-ERp57 complexes had different activities in enhancing peptide loading of MHC class I molecules. We seek to better understand the nature of the differences. We also seek to understand the role of careticulin in tapasin-assisted MHC class I assembly. Although all MHC class I molecules appear to follow the same assembly route within the ER, closely related HLA-B allotypes differ dramatically in their intrinsic rates of assembly and ER exit. In the studies proposed here, we seek to classify high frequency HLA-B alleles as rapid or slow trafficking, and to also examine the functional consequences of rapid or slow trafficking upon antigen presentation and disease progression. It is our hypothesis that the trafficking phenotypes can impact both the CTL response and the NK cell response, which will be further examined. Together, these studies will allow for a better understanding of the different steps of the MHC class I assembly route, and will contribute to the development of more effective strategies to enhance CTL responses in infection and cancer.

Public Health Relevance

An understanding of the substrate interaction site of TAP, and of potential resting state conformations of TAP (inactive conformations) will be important for future designs of TAP inhibitors that could be of use in settings of transplantation and autoimmunity, and additionally in the design of inhibitors of other ABC transporters to overcome drug/antibiotic resistance. A better understanding of the mechanism of tapasin function could lead to new strategies for enhancing assembly of specific immunogenic peptides with MHC Class I molecules in infection and cancer. Finally, an understanding of how trafficking differences between HLA-B allotypes impact their antigen presenting ability will be important for better elucidating the effects of different HLA antigens on disease susceptibility, resolution, and progression.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
Research Project (R01)
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Cellular and Molecular Immunology - B Study Section (CMIB)
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Gondre-Lewis, Timothy A
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University of Michigan Ann Arbor
Schools of Medicine
Ann Arbor
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Rizvi, Syed Monem; Salam, Nasir; Geng, Jie et al. (2014) Distinct assembly profiles of HLA-B molecules. J Immunol 192:4967-76
Rizvi, Syed Monem; Del Cid, Natasha; Lybarger, Lonnie et al. (2011) Distinct functions for the glycans of tapasin and heavy chains in the assembly of MHC class I molecules. J Immunol 186:2309-20
Thammavongsa, Vilasack; Schaefer, Malinda; Filzen, Tracey et al. (2009) Assembly and intracellular trafficking of HLA-B*3501 and HLA-B*3503. Immunogenetics 61:703-16
Rizvi, Syed Monem; Raghavan, Malini (2006) Direct peptide-regulatable interactions between MHC class I molecules and tapasin. Proc Natl Acad Sci U S A 103:18220-5
Perria, Christopher L; Rajamanickam, Vijayalakshmi; Lapinski, Philip E et al. (2006) Catalytic site modifications of TAP1 and TAP2 and their functional consequences. J Biol Chem 281:39839-51
Thammavongsa, Vilasack; Raghuraman, Gayatri; Filzen, Tracy M et al. (2006) HLA-B44 polymorphisms at position 116 of the heavy chain influence TAP complex binding via an effect on peptide occupancy. J Immunol 177:3150-61
Thammavongsa, Vilasack; Mancino, Laura; Raghavan, Malini (2005) Polypeptide substrate recognition by calnexin requires specific conformations of the calnexin protein. J Biol Chem 280:33497-505
Yang, Tianyu; Lapinski, Philip E; Zhao, Haotian et al. (2005) A rare transporter associated with antigen processing polymorphism overpresented in HLAlow colon cancer reveals the functional significance of the signature domain in antigen processing. Clin Cancer Res 11:3614-23
Lapinski, Philip Edward; Raghuraman, Gayatri; Raghavan, Malini (2003) Nucleotide interactions with membrane-bound transporter associated with antigen processing proteins. J Biol Chem 278:8229-37
Raghuraman, Gayatri; Lapinski, Philip Edward; Raghavan, Malini (2002) Tapasin interacts with the membrane-spanning domains of both TAP subunits and enhances the structural stability of TAP1 x TAP2 Complexes. J Biol Chem 277:41786-94

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