The ins and outs of intracellular peptides and antigen presentation by MHC class I molecules.

MHC class I molecules present small intracellular generated fragments to the outside surveying immune system. This is the result of a series of biochemical processes involving biosynthesis, degradation, translocation, intracellular transport, diffusion, and many more. Critical intermediates and end products of this cascade of events are peptides. The peptides are generated by the proteasome, degraded by peptidases unless transported into the ER where another peptidase and MHC class I molecules are waiting. Unless peptides bind to MHC class I molecules, they are released from the ER and enter the cytoplasm by a system resembling the ERAD pathway in many aspects. The cycle of peptides over the ER membrane with the proteasome at the input site and peptidases or MHC class I molecules on the output site are central in the MHC class I antigen presentation pathway and this review.