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Animated version (Shockwave Flash)
Stephen Man
Figure 3. Degradation and transport of antigens that bind major histocompatibility complex (MHC) class I molecules. (a) In an antigen-presenting cell (APC), newly synthesised MHC class I molecules bind to calnexin (Cx), which retains them in a partially folded state in the endoplasmic reticulum (ER). (b) Binding of MHC class I molecules to b2 microglobulin (b2m) displaces Cx and allows binding of chaperonin proteins (calreticulin and tapasin; not shown). (c) The MHC class I–b2m complex binds to the TAP complex (TAP1–TAP2), which awaits the delivery of peptides. (d) Peptides (e.g. from antigens) are formed from the degradation of cytosolic proteins (‘self’-, pathogen- and tumour-derived proteins in the cytoplasm). (e) These are degraded by proteasomes into (f) short peptides. (g) Peptides are transported into the ER by the TAPs, where they meet the MHC class I–b2m complex (h). This peptide binding in the antigenic groove of the MHC stablises the structure of the MHC class I molecule and (i) releases the TAP complex. (j) The fully folded MHC class I molecule with its peptide is transported to the cell surface via the Golgi apparatus. (k) Recognition of the MHC class I–peptide complex by the T-cell receptor (TCR) of an antigen-specific (CD8+, CD3+) cytotoxic T lymphocyte (CTL) takes place and (l) a signal transduction event activates effector functions in the MHC-class-I-restricted T cell; this requires co-stimulation to occur (not shown) (fig003smc).
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