Journal of Molecular Biology
Regular articleSequence signals for generation of antigenic peptides by the proteasome: implications for proteasomal cleavage mechanism1
Introduction
Cytotoxic T lymphocytes recognize short peptides of eight to ten amino acid residues, the processing products of protein antigens, presented on the surface of an antigen presenting cell. The main processing pathway involves degradation of cytosolic proteins, mostly by the proteasome, and transport of the degradation products by the transporter associated with antigen processing (TAP) to the endoplasmic reticulum (ER), where peptide binding to major histocompatibility complex (MHC) class I molecules takes place. This is followed by trafficking of the MHC-peptide complexes to the cell surface for presentation to cytotoxic T lymphocytes (reviewed by Lehner and Cresswell 1996, York and Rock 1996, Pamer and Cresswell 1998). The specificity of each of these processing stages influences the selection of T-cell epitopes along a protein sequence. Here, we concentrate on the first processing stage, the protein cleavage by the proteasome, attempting to identify sequence signals that may play a role in the determination of protein cleavage sites.
There is accumulating evidence suggesting that the proteasome is involved in the generation of most antigenic peptides (reviewed by Koopmann et al 1997, Pamer and Cresswell 1998, Rock and Goldberg 1999). The proteasome, a multimeric proteinase that is ubiquitous in all eukaryotic cells, is the central enzyme of protein degradation in both the cytosol and nucleus (for a review, see Baumeister et al., 1998). The proteolytic core of this complex is formed by the 20 S proteasome, whose general architecture is conserved from archaebacteria to eukaryotes Lowe et al 1995, Groll et al 1997, but the identity of its subunits and their refined structure differ. The 20 S proteasome is composed of two copies of 14 subunits organized in four rings. In the yeast 20 S proteasome the outer two rings contain subunits of the form (α1…α7) and the inner two rings, where the active sites reside, contain subunits of the form (β1…β7).
Three major specific peptidase activities attributed to three active sites have been defined (e.g. Orlowski et al 1993, Groll et al 1997, Heinermeyer et al 1997, Dick et al 1998, Nussbaum et al 1998): trypsin-like activity for subunit β2 (cleavage after basic residues); chymotrypsin-like activity for β5 (cleavage after hydrophobic residues); and peptidylglutamyl-peptide hydrolytic activity (PGPH) for β1 (cleavage after acidic residues). Recent studies with yeast 20 S proteasome have indicated that the β2 and β1 also contribute substantially to the chymotrypsin-like activity Dick et al 1998, Nussbaum et al 1998. In vertebrates there are three other β subunits that are induced by increased levels of interferon-γ upon induction of an immune response: β1i (LMP2), β5i (LMP7), and β2i (MECL1) that replace β1, β5 and β2 (Y, X, and Z in human), respectively (reviewed by Pamer & Cresswell, 1998). Assembly of this “immuno-proteasome” affects the hierarchy of proteasomal cleavage, enhancing cleavage after basic and hydrophobic residues and inhibiting cleavage after acidic residues (e.g. Gaczynska et al., 1994). This is in accord with the type of residues required at the C terminus of antigenic peptides for binding to the F pocket of MHC class I molecules (Madden, 1995).
From studies of the sequences of degradation products it has become clear that the nature of the peptidase activities by themselves cannot explain the cleavage specificity (e.g. Niedermann et al 1995, Niedermann et al 1996, Ehring et al 1996, Nussbaum et al 1998). In the studied cases, the amino acid residues at the P1 position † of many cleavage sites conformed to the peptidase activities described above, but other residues were also observed, indicating a large variation. Furthermore, residues that were cleaved preferentially at one sequence location behaved differently at other positions, suggesting that the sequence context of the cleavage sites may also play a role. Indeed, Nussbaum et al. (1998) have recently demonstrated some correlations between the amino acid residues at position P1 of a cleavage site and residues in its vicinity, in a span of ± five residues. Holzhutter et al. (1999) performed a statistical analysis on the flanking regions of a small sample of sequences of degradation products reported in the literature, and also described correlations between the type of residues at P1 and specific residues in nearby positions. However, the nature of the correlations were different than those reported by Nussbaum et al. (1998). Also, several experimental studies have indicated the effect of changes in the peptide flanking residues on antigen presentation (e.g. Bergmann et al 1994, Shastri et al 1995, Bergmann et al 1996, Yellen-Shaw et al 1997, Vijh et al 1998). Still, the precise location of the residues which affected presentation has been somewhat controversial. The inconsistencies in these results are probably due to the small number of experimentally studied sequences, which may be insufficient for deducing general rules.
An alternative approach would be to use the available information on naturally processed peptide sequences that were eluted from MHC molecules, and analyze their flanking residues. These peptides were cleaved by the proteasome, and if specific cleavage signals exist they should present them at their termini and/or flanking sequences. Therefore, the question that we address here is, can we identify significant signals at the termini and flanking regions of naturally processed peptides that can be attributed to proteasomal cleavage? A first attempt in this direction was carried out by Niedermann et al. (1996), who experimentally determined the amino acid preferences at the C and N termini and flanking residues of degradation products, and compared these results with the frequency distribution of amino acids at the terminal and immediate flanking positions of a set of 134 MHC class I-bound peptides. They demonstrated strong conformity in the distribution of residues at the C termini of the two sets and a weaker similarity between the distributions of residues at the N termini. Based on these results they suggested that the proteasome is frequently involved in the generation of the C termini of antigenic peptides, and in the generation of some but not all of their N termini.
Here, we have carefully organized a database of 286 naturally processed peptide sequences from 215 different proteins, and have applied systematic computational analyses to search for potential cleavage signals at the amino and carboxy-terminal residues, and in the flanking regions of the peptides, spanning 50 residues on each side. The signals that we have identified in a completely objective way are consistent with reported experimental results and emphasize the importance of position P1′ in determining the cleavage specificity of both the N and C termini. We propose a model for proteasomal cleavage mechanism and generation of antigenic peptides that incorporates the computer analysis implications with those derived from experimental results. Our results and conclusions can serve as the basis for a computer algorithm that will predict the boundaries of the initial degradation products along a protein antigen.
Section snippets
N-terminal flanking region
The 274 N-terminal flanking fragments were divided randomly into two equal-size groups (see Methods). In each group the sequences were aligned by the first (PN) peptide residue (refer to Figure 1 for nomenclature of sequence positions). First, one group of 137 sequences was analyzed. The amino acid frequency distribution at each of the N1 to N50 positions was compared to the amino acid frequency distribution in the data set of non-homologous source proteins (background frequency distribution)
Discussion
Unraveling the rules for generation of antigenic peptides by the proteasome is essential for understanding the selection of immunodominant T-cell epitopes along the sequence of a protein antigen. Availability of a quantitative measure that reflects the likelihood of cleavage between any pair of residues within a given sequence may enable the prediction of the protein cleavage pattern. This may provide the means for detection of potential protein fragments that will be further processed as
Organization of peptide sequence database
Peptide sequences associated with MHC class I of mouse and/or human were taken from a compilation of MHC ligands which includes known T-cell epitopes and naturally processed peptides eluted from MHC molecules (Rammensee et al., 1995; see http://www.uni-tuebingen.de/uni/kxi). In the current analysis only naturally processed peptides longer than six amino acid residues were included. Each peptide sequence was verified with the original paper, keeping the peptides that were individually sequenced,
Acknowledgements
We are grateful to Klaus Eichmann for valuable and inspiring discussions. We thank Norman Grover for statistical consultation, Jutta Bachmann for her help in the data organization, and Ora Schueler-Furman for helpful comments on the manuscript. This work has benefited from valuable discussions with Robert Huber, Wolfgang Baumeister, Robert Tampe, Michelle Groll, and Alfred Goldberg. The study was supported by grants from the US-Israel Binational Science Foundation, and from the Israel Cancer
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Edited by W. Baumeister