Journal of Molecular Biology
Regular articleAn algorithm for the prediction of proteasomal cleavages1
Introduction
Proteasomes are cytosolic multisubunit proteases which are involved in cell cycle control, transcription factor activation and the generation of peptide ligands for MHC I molecules (for reviews, see Baumeister et al 1998, Rock and Goldberg 1999, Uebel and Tampe 1999). They exist in several forms: the proteolytically active core complexes, or 20 S proteasomes, and, when associated with the ATP-dependent 19 S cap complexes, the larger 26 S proteasomes that are able to recognize proteins marked by ubiquitin for proteasomal degradation Jentsch and Schlenker 1995, Hershko and Ciechanover 1998. Another protein complex known to associate with the 20 S core particle is PA28, the 11 S regulator (Ahn et al., 1995), which was shown to improve the yield of antigenic peptides Groettrup et al 1996, Dick et al 1996.
Eukaryotic 20 S proteasomes consist of four stacked rings (overall stoichiometry α7β7β7α7), each consisting of seven different subunits (Groll et al., 1997). Each of the two inner β-rings carries three catalytically active sites on its inner surface. Their proteolytic specificities have been described as chymotrypsin-like (cleaving after large, hydrophobic AAs), trypsin-like (cleaving after basic AAs) and peptidyl-glutamyl-peptide-hydrolyzing (cleaving after acidic AAs) (for a review, see Uebel & Tampe, 1999). Strings of unfolded proteins are thought to be inserted into the cylinder and to be cut into pieces by the active sites; the resulting peptide fragments are then released into the cytosol. Functionally, proteasomal protein degradation is believed to proceed from one substrate end to the other (“processively”), without the release of large degradation intermediates Akopian et al 1997, Nussbaum et al 1998, Kisselev et al 1999a.
In vertebrate cells, some of the proteolytic fragments produced by the proteasome are fed into the antigen processing machinery. Since peptide presentation by MHC I molecules at the cell surface is an intrinsic requirement for the ability of the immune system to eradicate virus-infected or transformed cells Rammensee et al 1993, Pamer and Cresswell 1998, it is of general interest to know exactly how the proteasome is involved in this process. Proteasomal cleavage specificity has been assessed by in vitro digestion experiments using either tri- or tetrapeptides with fluorogenic leaving groups Kuckelkorn et al 1995, Heinemeyer et al 1997, Arendt and Hochstrasser 1997, peptides of 15–40 AAs Boes et al 1994, Niedermann et al 1995, Niedermann et al 1996, Dick et al 1998, or denatured proteins Dick et al 1991, Dick et al 1994, Kisselev et al 1998, Kisselev et al 1999a as substrates. We analyzed the cleavage preferences of yeast wild-type and mutant proteasomes in a non-modified protein (Nussbaum et al., 1998) †. Using statistical analysis of cut sites, it was possible for the first time to determine so-called cleavage motifs, i.e. the preferred sequences around cleavage sites, for the three active β-subunits of yeast proteasomes.
In order to apply this cleavage site information to any possible proteasome substrate, an automated prediction device is needed. Such devices already exist for the binding of peptides to MHC I molecules (Rammensee et al., 1997) and have been described for peptide transport by the transporter associated with antigen processing (TAP) (Daniel et al., 1998). However, devices for the prediction of proteasomal cleavages are only at the beginning of their development. Recently, published peptide cleavage data were used to develop a prediction algorithm (Holzhutter et al., 1999), which reproduced its training data with 93 % and predicted non-training cleavages in one peptide substrate with 80 % accuracy. For different AAs in the P1 position ‡, cleavage motifs spanning up to 13 AAs were calculated by the algorithm. However, it has recently been shown that the three different proteolytic activities of eukaryotic proteasomes exhibit overlapping specificities Dick et al 1998, Nussbaum et al 1998. We therefore planned to generate a prediction device that does not rely on various motifs for different P1-AAs, but reflects a combination of the cutting of three active sites. This should mirror the experimental situation more efficiently where observed cleavages arise from a mixture of three different, partly overlapping proteasomal cleavage specificities. More importantly, we wanted our approach to be based on a more homogeneous set of training data, possibly generated under identical conditions. A device for proteasomal cleavage prediction would take us one step further in predicting the selection of CTL epitopes presented on MHC I by the three well-described “funnels of specificity”: proteasome cleavages, TAP transport, MHC I binding.
To this end, we developed a network-based model for proteasome cleavages, trained by an evolutionary algorithm on cleavages in protein and some peptide substrates (Nussbaum et al 1998, Niedermann et al 1995, Niedermann et al 1996, Niedermann et al 1997; our unpublished results). Our program performed significantly better for experimental data than for randomly positioned cuts, suggesting that it extracts rules inherent to proteasomal cleavages from training data. Besides, it reproduced the training data with very high level of accuracy (98–100 %). The parameters of the proteasome model largely reflect the roles of particular AAs in the cleavage motifs determined experimentally. Prediction of non-training cleavages was tested for some peptide substrates containing known MHC I ligands. The results indicate our approach to represent a promising starting point for refined algorithms for proteasomal cleavage and fragment prediction.
Section snippets
The experimental training data
Before designing a prediction tool, it was necessary to inspect carefully the experimental training data §. In brief, the experimental setup was as follows: purified 20 S proteasomes were co-incubated with a protein substrate (enolase, 436 AAs), enolase fragments were generated by the proteolytic action of the proteasome, and these fragments were identified by biochemical methods, allowing us to locate proteasomal cleavage sites. Thus, the experimental data consist of fragments and cleavage
Reproduction performance
Our method was applied to experimental cleavage data generated by different 20 S proteasomes (yeast wild-type, single mutants, double mutant, human wild-type) in the substrate enolase (Nussbaum et al., 1998; our unpublished results), yielding different model proteasome “species” (1–8; Table 1).
Initially, the double mutant data were utilized because of their “cleavage homogeneity”. The decision to cut was based on information from the octameric interval P4,..., P4′, i.e. k = m = 4. An initial
Discussion
Here we present a simple (one-layer) network capable of generating proteasomal cleavages in any given AA sequence. For training the network, goal functions counting missing and superfluous cuts (and cleavage probability in case of overlapping fragments in a refined model) were used, together with a stochastic, hill-climbing optimization process. Thus, parameter sets were found which reproduced the training data (cuts performed by 20 S proteasomes in enolase plus some cuts in ovalbumin peptides)
Acknowledgements
This work was supported by grants awarded by the Deutsche Forschungsgemeinschaft (Leibnizprogramm to H.-G.R. (Ra369/4–1); Schi 301/2–1 and Sonderforschungsbereich 510, C1 to H.S.) and the European Union (Biotech 95–1627). We thank Lynne Yakes for critically reading the manuscript.
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Edited by R. Huber
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These authors contributed equally to the work.
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Present address: T. P. Dick, Section of Immunobiology, Howard Hughes Medical Institute, Yale University School of Medicine, 310 Cedar Street, PO Box 208011, New Haven, CT 06520-8011, USA.