2 DISTINCT PROTEOLYTIC PROCESSES IN THE GENERATION OF A MAJOR HISTOCOMPATIBILITY COMPLEX CLASS I-PRESENTED PEPTIDE

Although cellular proteins degraded by proteasomes are the source of m ost antigenic peptides presented on major histocompatibility complex c lass I molecules, it is unknown whether the eight-to nine-residue pept ides that fit in the binding groove of class I molecules are directly produced by proteasomes alone in vivo, If the eight-residue peptide SI INFEKL from chicken ovalbumin is extended by one or several residues a t its C terminus and microinjected into cells or expressed from a mini gene, it is processed and presented on major histocompatibility comple x class I, However, processing and presentation are inhibited by prote asome inhibitors, such as lactacystin, In contrast, when SIINFEKL is e xtended by 2 to 25 residues at its N terminus, its presentation is not blocked by proteasome inhibitors, N-terminal processing also can occu r when the extended peptide is cotranslationally inserted into the end oplasmic reticulum. Thus, two different proteolytic steps in the gener ation of an chicken ovalbumin-presented peptide can be distinguished. Cleavage by the proteasome defines the proper C terminus, whereas dist inct peptidase(s) in the cytosol or endoplasmic reticulum may generate the appropriate N terminus from extended peptides.