Major histocompatibility complex (MHC) class I molecules bind peptides
that are delivered from the cytosol into the endoplasmic reticulum by
the MHC-encoded transporter associated with antigen processing (TAP).
Peptide capture by immature heterodimers of class I heavy chains and
beta(2)-microglobulin may be facilitated by their physical association
with TAP. A genetic defect in a human mutant cell line causes the com
plete failure of diverse class I heterodimers to associate with TAP. T
his deficiency impairs the ability of the class I heterodimers to effi
ciently capture peptides and results from loss of function of an unide
ntified gene or genes linked to the MHC.