Post-Endoplasmic Reticulum Rescue of Unstable MHC Class I Requires Proprotein Convertase PC7

Abstract

The function of the peptide-loading complex (PLC) is to facilitate loading of MHC class I (MHC I) molecules with antigenic peptides in the endoplasmic reticulum and to drive the selection of these ligands toward a set of high-affinity binders. When the PLC fails to perform properly, as frequently observed in virus-infected or tumor cells, structurally unstable MHC I peptide complexes are generated, which are prone to disintegrate instead of presenting Ags to cytotoxic T cells. In this study we show that a second quality control checkpoint dependent on the serine protease proprotein convertase 7 (PC7) can rescue unstable MHC I, whereas the related convertase furin is completely dispensable. Cells with a malfunctioning PLC and silenced for PC7 have substantially reduced MHC I surface levels caused by high instability and significantly delayed surface accumulation of these molecules. Instead of acquiring stability along the secretory route, MHC I appears to get largely routed to lysosomes for degradation in these cells. Moreover, mass spectrometry analysis provides evidence that lack of PLC quality control and/or loss of PC7 expression alters the MHC I-presented peptide profile. Finally, using exogenously applied peptide precursors, we show that liberation of MHC I epitopes may directly require PC7. We demonstrate for the first time an important function for PC7 in MHC I-mediated Ag presentation.

Peptide transporter TAP mediates between competing antigen sources generating distinct surface MHC class I peptide repertoires

“…However, processing by yet other proteolytic systems deliver peptides that are presented in a TAP-independent way. The above-mentioned SP and SPP proteases produce such TAPindependent peptides within the ER and proprotein convertases like PC7 and furin have been shown to facilitate TAP-independent presentation in the secretory route [29][30][31][32]. Interestingly, our preliminary data show that presentation of the Trh4 peptide is independent of these known enzyme systems, indicating that yet other pathways exist.…”

“…Peptides for which MHC-I binding and stability were analyzed were Trh4 379-387 (MCLRMTAVM), LCMV gp33 [33][34][35][36][37][38][39][40][41][42] [20][21][22][23][24][25][26][27][28] (TNLLNDRVL) and gp100 [25][26][27][28][29][30][31][32][33] (EGSRNQDWL). RMA-S cells were cultured for 2 days at 261C to accumulate peptide receptive MHC-I molecules on the cell surface [60].…”

“…Indeed, signal sequences are liberated by the combined action of signal peptidase (SP) and signal peptide peptidase (SPP) and are directly available for loading into MHC-I [28,29]. A second characterized processing pathway that bypasses TAP is active in the secretory route and is mediated by members of the proprotein convertase (PC) family, like furin [30][31][32]. This enzyme is located in the trans-Golgi network and mediates the proteolytic maturarion of many proproteins, e.g.…”

Characterization of a Proteasome and TAP-independent Presentation of Intracellular Epitopes by HLA-B27 Molecules

“…It has been recently demonstrated that in TAP-defective cells, HLA-class I molecules can exchange peptides in post-ER vesicles such as in Trans Golgi network (TGN) (19), We have also shown that an HLA-B27-restricted epitope from Influenza virus can be cross-presented to CD8 T cells when exogenously carried to the TGN by chimeric proteins. In this case the presentation was proteasome and TAP-independent.…”

Disruption of the expression of the proprotein convertase PC7 reduces BDNF production and affects learning and memory in mice

“…The tail also contains two cysteine residues, Cys 699 and Cys 704 , which are palmitoylated (3,4,6), that may assist in this process. Confocal and electron microscopy studies have revealed that PC7 localizes to vesicles located immediately beneath the plasma membrane (4,7). No soluble shed forms of PC7 have been detected.…”