Peptide splicing by the proteasome

Abstract

Edited by Peter CresswellThe proteasome is the major protease responsible for the production of antigenic peptides recognized by CD8؉ cytolytic T cells (CTL). These peptides, generally 8 -10 amino acids long, are presented at the cell surface by major histocompatibility complex (MHC) class I molecules. Originally, these peptides were believed to be solely derived from linear fragments of proteins, but this concept was challenged several years ago by the isolation of anti-tumor CTL that recognized spliced peptides, i.e. peptides composed of fragments distant in the parental protein. The splicing process was shown to occur in the proteasome through a transpeptidation reaction involving an acyl-enzyme intermediate. Here, we review the steps that led to the discovery of spliced peptides as well as the recent advances that uncover the unexpected importance of spliced peptides in the composition of the MHC class I repertoire.The immune system constantly monitors cellular integrity to restrain tumor development or viral infections. CD8 ϩ cytolytic T lymphocytes (CTL) 4 are essential players in this process, because of their ability to recognize and destroy cells expressing tumoral or viral proteins. CTL indeed recognize peptides derived from these proteins and displayed at the cell surface by major histocompatibility complex (MHC) class I molecules. Peptides recognized by CTL are produced in the cytosol by the proteasome, a large protease complex responsible for the bulk of cellular protein degradation (1). A fraction of the peptides released by the proteasome is thus transferred into the lumen of the ER by a specialized transporter called TAP (transporter associated with antigen processing) (2), where they can be further trimmed by ER-resident amino peptidases (ERAP1 and ERAP2) (3-5) and loaded onto MHC class I molecules. Stable MHC-peptide complexes then exit the ER and migrate through the secretory pathway to reach the cell surface, where they will be displayed for CTL recognition (6). For a number of years, antigenic peptides recognized by CTL were believed to only derive from linear fragments of cellular proteins. This concept was challenged by the identification of several antigenic peptides, composed of two peptide fragments that were originally distant in the parental protein but are assembled together by the creation of a new peptide bond (7-13). This process, called peptide splicing, was shown to take place in the proteasome (8 -10). In this minireview, we will recapitulate the discovery of this phenomenon and the recent developments that highlight the unforeseen importance of peptide splicing in the establishment of the MHC class I peptide repertoire.

HIPs and HIP-reactive T cells

“…One mechanism of HIP formation may occur through reversed proteolysis (Fig. a), as previously implicated in the generation of spliced peptides by the proteasome . During a reversed proteolytic reaction, a peptide bond is cleaved by a protease.…”

“…Efforts to identify HIPs and the closely related class of spliced peptides can be mutually beneficial. Generation of spliced peptides by the proteasome and presentation of these peptides by MHC class I molecules has now been studied for more than a decade , but reports on efforts to broadly identify spliced peptides have only recently started to emerge. Because the HIPs being studied in the context of T1D are composed of fragments of peptides enclosed in beta cell secretory granules, they may be formed by a different mechanism (possible mechanisms of HIP formation will be discussed later in this review).…”

Shigellaeffector IpaH4.5 targets 19S regulatory particle subunit RPN13 in the 26S proteasome to dampen cytotoxic T lymphocyte activation

“…The 26S proteasome is essential for generation of antigenic peptides suitable for binding to MHC class I, a process termed PCPS (Platteel, Liepe, van Eden, Mishto, & Sijts, ; Vigneron, Ferrari, Stroobant, Abi Habib, & Van den Eynde, ). The peptides generated by PCPS are transported together with MHC class I to the cell surface, where they are presented to CD8+ T cells, resulting in induction of CTLs (Platteel et al, ).…”

“…, the level of Rpn13 decreased, whereas the level of ubiquitinated proteins increased, in an IpaH4.5 ligase-dependent manner in the infected BMDMs, as well as non-myeloid HCT116 cells (Figure 5c). Taken together, these data demonstrate that IpaH4.5mediated RPN13 degradation dampens the function of the RP in the 26S proteasome during Shigella infection in both myeloid and nonmyeloid cells.2.7 | Dysfunction of the RP in the 26S proteasome induced by IpaH4.5 affects CD8 + T cell primingThe 26S proteasome is essential for generation of antigenic peptides suitable for binding to MHC class I, a process termed PCPS(Platteel, Liepe, van Eden, Mishto, & Sijts, 2017;Vigneron, Ferrari, Stroobant, Abi Habib, & Van den Eynde, 2017). The peptides generated by PCPS are transported together with MHC class I to the cell surface, where…”

Islet stress, degradation and autoimmunity

“…Condensation, describes the formation of peptide bond de novo. 50 While theoretically, both the constitutive and induced β subunits may perform transpeptidation or condensation, some SP100 derivatives are more pronounced in the presence of the immunoproteasome, 51 and spliced gp100(47-52)- (40)(41)(42) shown to occur by condensation by the action of β5i subunit. 50 In autoimmune disease these processes are poorly investigated.…”