A role for calnexin in the assembly of the MHC class I loading complex in the endoplasmic reticulum

Heterodimers of MHC class I glycoprotein and beta (2)-microglobulin (beta ( 2)m) bind short peptides in the endoplasmic reticulum (ER), Before peptide binding these molecules form part of a multisubunit loading complex that al so contains the two subunits of the TAP, the transmembrane glycoprotein tap asin, the soluble chaperone calreticulin, and the thiol oxidoreductase ERp5 7, We have investigated the assembly of the loading complex and provide evi dence that after TAP and tapasin associate with each other, the transmembra ne chaperone calnexin and ERp57 bind to the TAP-tapasin complex to generate an intermediate, These interactions are independent of the N-linked glycan of tapasin, but require its transmembrane and/or cytoplasmic domain. This intermediate complex binds MHC class I-beta (2)m dimers, an event accompani ed by the loss of calnexin and the acquisition of calreticulin, generating the MHC class I loading complex. Peptide binding then induces the dissociat ion of MHC class I-beta (2)m dimers, which can be transported to the cell s urface.