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.