Abstract
Abstract Cellular peptides generated by proteasomal degradation of proteins in the cytosol and destined for presentation by MHC I are associated with several chaperones. Hsp70, hsp90, and the TCP1-ring complex have been implicated as important cytosolic players for chaperoning these peptides. In this study we report that gp96 and calreticulin are essential for chaperoning peptides in the endoplasmic reticulum. Importantly we demonstrate that cellular peptides are transferred sequentially from gp96 to calreticulin and then to MHC I forming a relay line. Disruption of this relay line by removal of gp96 or calreticulin prevents the binding of peptides by MHC I and hence presentation of the MHC 1-peptide complex on the cell surface. Our results are important for understanding how peptides are processed and trafficked within the endoplasmic reticulum before exiting in association with MHC I heavy chains and beta2-microglobulin as a trimolecular complex.
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