The multistep process that culminates in major histocompatibility complex (MHC) class I presentation of foreign or self-peptides begins in the last phases of protein catabolism. Although the individual roles of many key molecules—such as proteasomes, the transporter associated with antigen processing, and various endoplasmic reticulum chaperones—have recently been elucidated, there still remain many questions regarding processing of proteins into MHC class I bound peptides. This review summarizes the recent developments in antigen processing for MHC class I molecules, with a focus on how proteins are believed to be sampled and selected for degradation.
