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Abstract
Tapasin forms a bridge between TAP (transporters associated with antigen processing)
and MHC class I molecules and plays a critical role in class I assembly. In its absence,
TAP and class I do not associate, and class I cell surface expression is reduced.
We now identify two independent functions for tapasin. Tapasin increases TAP levels
and allows more peptide to be translocated to the endoplasmic reticulum. Furthermore,
when expressed in the tapasin-negative .220 cell line, recombinant soluble tapasin
retains its association with class I and restores class I cell surface expression
and function, even though it no longer binds TAP or increases TAP levels. This finding
suggests that the association of tapasin with class I is sufficient to facilitate
loading and assembly of class I molecules.