Distinct functions of tapasin revealed by polymorphism in MHC class I peptide loading

Peptide assembly,vith class I molecules is orchestrated by multiple chapero nes including tapasin, which bridges class I molecules with the TAP and is critical for efficient Ag presentation. In this paper, we show that, althou gh constitutive levels of endogenous murine tapasin apparently are sufficie nt to form stable and long-lived complexes between the human HLA-B*4402 (B* 4402) and mouse TAP proteins, this does not result in normal peptide loadin g and surface expression of B*4402 molecules on mouse APC. However, increas ed expression of murine tapasin, but not of the human TAP proteins, does re store normal cell surface expression of B*4402 and efficient presentation o f viral Ags to CTL, High levels of soluble murine tapasin, which do not bri dge TAP and class I molecules, still restore normal surface expression of B *4402 in the tapasin-deficient human cell line 721.220, These findings indi cate distinct roles for tapasin in class I peptide loading, First, tapasin- mediated bridging of TAP-class I complexes, which despite being conserved a cross the human-mouse species barrier, is not necessarily sufficient for pe ptide loading. Second, tapasin mediates a function which probably involves stabilization of empty class I molecules and which is sensitive to structur al compatibility of components within the loading complex. These discrete f unctions of tapasin predict limitations to the study of HLA molecules acros s some polymorphic and species barriers.