The nature of the MHC class I peptide loading complex.

Peptide binding to major histocompatibility complex (MHC) class I molecules occurs in the endoplasmic reticulum (ER). Efficient peptide binding requires a number of components in addition to the MHC class I-beta 2 microglobulin dimer (beta 2m). These include the two subunits of the transporter associated with antigen presentation (TAP1 and TAP2), which are essential for introducing peptides into the ER from the cytosol, and tapasin, an MHC-encoded membrane protein. Prior to peptide binding, MHC class I-beta 2m dimers form part of a large multisubunit ER complex which includes TAP and tapasin. In addition to these specialized components two soluble 'house-keeping' proteins, the chaperone calreticulin and the thiol oxidoreductase ERp57, are also components of this complex. Our current understanding of the nature and function of the MHC class I peptide loading complex is the topic of this review.