ClassLiberate C-Terminal Peptides for MHC New Role of Signal Peptide Peptidase

The signal peptide peptidase (SPP) is an intramembrane cleaving aspartyl protease involved in release of leader peptide remnants from the endoplasmic reticulum membrane, hence its name. We now found a new activity of SPP that mediates liberation of C-terminal peptides. In our search for novel proteolytic enzymes involved in MHC class I (MHC-I) presentation, we found that SPP generates the C-terminal peptide-epitope of a ceramide synthase. The display of this immunogenic peptide–MHC-I complex at the cell surface was independent of conventional processing components like proteasome and peptide transporter TAP. Absence of TAP activity even increased the MHC-I presentation of this Ag. Mutagenesis studies revealed the crucial role of the C-terminal location of the epitope and " helix-breaking " residues in the transmembrane region just upstream of the peptide, indicating that SPP directly liberated the minimal 9-mer peptide. Moreover, silencing of SPP and its family member SPPL2a led to a general reduction of surface peptide–MHC-I complexes, underlining the involvement of these enzymes in Ag processing and presentation. A ll nucleated cells of our body display a representative selection of the cellular proteome in MHC class I (MHC-I) molecules at their surfaces. This wide array of peptides serves as ligands for CD8 + T cells for cellular immunity. The process of peptide presentation starts with the proteolysis of aged or misfolded proteins, a process mainly executed by the multi-catalytic action of proteasomes. The generated peptide sequences are translocated from the cytosol into the endoplasmic reticulum (ER) by the TAP peptide transporters, where they are assembled with different MHC-I molecules with help from members of the peptide loading complex, including calreticulin, tapasin, and ERp57. Stable peptide–MHC-I complexes are routed to the cell surface. This proteasome–TAP pathway is considered as the conventional processing route used to generate and shuttle the majority of peptides (1–3). Additional players, like tripeptidyl peptidase II and nardilysin, are responsible to further shorten proteasomal intermediates or, sometimes, to directly generate minimal peptide sequences that fit in MHC-I molecules (4, 5). Although these central elements of the conventional Ag processing pathway are important for the mainstream of peptides, substantial MHC-I Ag presentation is still detectable on cells in which the function of key components is compromised. For example, deficiency in the peptide transporter TAP, which can be regarded as a bottleneck in the pathway, results in a decreased surface MHC-I, but the residual peptide repertoire is sufficient to generate a diverse …