Direct peptide-regulatable interactions between MHC class I molecules and tapasin.
Tapasin (Tpn) has been implicated in multiple steps of the MHC class I assembly pathway, but the mechanisms of function remain incompletely understood. Using purified proteins, we could demonstrate direct binding of Tpn to peptide-deficient forms of MHC class I molecules at physiological temperatures. Tpn also bound to M10.5, a pheromone receptor-associated MHC molecule that has an open and empty groove and that shares significant sequence identity with class I sequences. Two types of MHC class I-Tpn complexes were detectable in vitro depending on the input proteins; those depleted in beta(2)m, and those containing beta(2)m. Both were competent for subsequent assembly with peptides, but the latter complexes assembled more rapidly. Thus, the assembly rate of Tpn-associated class I was determined by the conditions under which Tpn-MHC class I complexes were induced. Peptide loading of class I inhibited Tpn-class I-binding interactions, and peptide-depletion enhanced binding. In combination with beta(2)m, certain peptides induced efficient dissociation of preformed Tpn-class I complexes. Together, these studies demonstrate direct Tpn-MHC class I interactions and preferential binding of empty MHC class I by Tpn, and that the Tpn-class I interaction is regulated by both beta(2)m and peptide. In cells, Tpn is likely to be a direct mediator of peptide-regulated binding and release of MHC class I from the TAP complex.
Rizvi
Syed Monem
SM
Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109-0620, USA.
Raghavan
Malini
M
D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D015395:Histocompatibility Antigens Class I / Q000302:isolation & purification / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D010455:Peptides / Q000378:metabolism*; D011485:Protein Binding; D013696:Temperature; D001613:beta 2-Microglobulin / Q000378:metabolism
D015395:Histocompatibility Antigens Class I; D026901:Membrane Transport Proteins; D010455:Peptides; D001613:beta 2-Microglobulin; C107762:tapasin