1 Introduction

1.1 Motivation

Protease cleavages affect many vital physiological processes, and dysregulation of proteolytic activity is associated with a variety of pathological conditions. The field of degradomics is committed to provide insights about proteolytic events by incorporating techniques for identification and functional characterization of proteases and their substrates and inhibitors.

proteasy allows for batch identification of possible proteases for a set of substrates (protein IDs and peptide sequences), and may be an important tool in peptide-centric analyses of endogenously cleaved peptides.

This package utilizes data derived from the MEROPS database. The database is a manually curated knowledgebase with information about proteolytic enzymes, their inhibitors and substrates.

This document illustrates the functionality of proteasy through some use cases.

1.2 Package scope and limitations

Similarly to existing tools such as TopFind, and Proteasix, proteasy exists for the purpose of retrieving data about proteases by mapping peptide termini positions to known sites where a protease cleaves. Unlike the cleaver package, which allows for in-silico cleavage of amino acid sequences by specifying an enzyme, the functions in proteasy relies only on experimentally derived data to find proteases annotated to cleavage sites.

The proteasy package is limited to entries annotated in MEROPS. Moreover, proteasy currently does not allow for retrieval of proteolytic details for multiple organisms at once, or inter-organism events. The package does however provide annotation data for all organisms available in MEROPS.

The function findProtease will automatically map a peptide’s start- and end- positions in its host-protein’s sequence. When a protein sequence has repeated instances of a peptide sequence, proteasy will only match to the first instance in the protein sequence.

2 Installation

The package should be installed using as described below:

if (!requireNamespace("BiocManager", quietly = TRUE))
    install.packages("BiocManager")

BiocManager::install("proteasy")

The package can then be loaded with:

library("proteasy")

3 Usage

3.3 Find possible proteases for cleaved peptides

The function findProtease automatically maps the peptide sequences to the full-length protein sequence and obtains the start- and end-positions for the peptide. Then, the positions are searched against the MEROPs database and matches are returned.

# Create a vector of Uniprot IDs and corresponding peptide sequences

protein <- c("P02671", "P02671", "P68871",
             "P01011", "P68133", "P02461",
             "P0DJI8", "P0DJI8", "P0DJI8")
peptide <- c("FEEVSGNVSPGTR", "FVSETESR", "LLVVYPW",
             "ITLLSAL", "DSYVGDEAQS", "AGGFAPYYG",
             "FFSFLGEAFDGAR", "EANYIGSDKY", "GGVWAAEAISDAR")

# If we do not specify start position (start_pos) and end position
# (end_pos), the function will automatically assign these values by
# matching the provided peptide sequence against the full-length
# protein sequence.

res <- findProtease(protein = protein,
                    peptide = peptide,
                    organism = "Homo sapiens")
## Warning: replacing previous import 'utils::findMatches' by
## 'S4Vectors::findMatches' when loading 'AnnotationDbi'
# The resulting S4 object can be inspected in three ways;
# (to save space we show only the first six rows)

# 1. Display sequence data for the provided input:

substrates(res) |> head()
##              Substrate name Substrate (Uniprot)
## 1:   fibrinogen alpha chain              P02671
## 2:   fibrinogen alpha chain              P02671
## 3:  hemoglobin subunit beta              P68871
## 4: alpha-1-antichymotrypsin              P01011
## 5:  Serum amyloid A protein              P0DJI8
## 6:  Serum amyloid A protein              P0DJI8
##                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    Substrate sequence
## 1: MFSMRIVCLVLSVVGTAWTADSGEGDFLAEGGGVRGPRVVERHQSACKDSDWPFCSDEDWNYKCPSGCRMKGLIDEVNQDFTNRINKLKNSLFEYQKNNKDSHSLTTNIMEILRGDFSSANNRDNTYNRVSEDLRSRIEVLKRKVIEKVQHIQLLQKNVRAQLVDMKRLEVDIDIKIRSCRGSCSRALAREVDLKDYEDQQKQLEQVIAKDLLPSRDRQHLPLIKMKPVPDLVPGNFKSQLQKVPPEWKALTDMPQMRMELERPGGNEITRGGSTSYGTGSETESPRNPSSAGSWNSGSSGPGSTGNRNPGSSGTGGTATWKPGSSGPGSTGSWNSGSSGTGSTGNQNPGSPRPGSTGTWNPGSSERGSAGHWTSESSVSGSTGQWHSESGSFRPDSPGSGNARPNNPDWGTFEEVSGNVSPGTRREYHTEKLVTSKGDKELRTGKEKVTSGSTTTTRRSCSKTVTKTVIGPDGHKEVTKEVVTSEDGSDCPEAMDLGTLSGIGTLDGFRHRHPDEAAFFDTASTGKTFPGFFSPMLGEFVSETESRGSESGIFTNTKESSSHHPGIAEFPSRGKSSSYSKQFTSSTSYNRGDSTFESKSYKMADEAGSEADHEGTHSTKRGHAKSRPVRDCDDVLQTHPSGTQSGIFNIKLPGSSKIFSVYCDQETSLGGWLLIQQRMDGSLNFNRTWQDYKRGFGSLNDEGEGEFWLGNDYLHLLTQRGSVLRVELEDWAGNEAYAEYHFRVGSEAEGYALQVSSYEGTAGDALIEGSVEEGAEYTSHNNMQFSTFDRDADQWEENCAEVYGGGWWYNNCQAANLNGIYYPGGSYDPRNNSPYEIENGVVWVSFRGADYSLRAVRMKIRPLVTQ
## 2: MFSMRIVCLVLSVVGTAWTADSGEGDFLAEGGGVRGPRVVERHQSACKDSDWPFCSDEDWNYKCPSGCRMKGLIDEVNQDFTNRINKLKNSLFEYQKNNKDSHSLTTNIMEILRGDFSSANNRDNTYNRVSEDLRSRIEVLKRKVIEKVQHIQLLQKNVRAQLVDMKRLEVDIDIKIRSCRGSCSRALAREVDLKDYEDQQKQLEQVIAKDLLPSRDRQHLPLIKMKPVPDLVPGNFKSQLQKVPPEWKALTDMPQMRMELERPGGNEITRGGSTSYGTGSETESPRNPSSAGSWNSGSSGPGSTGNRNPGSSGTGGTATWKPGSSGPGSTGSWNSGSSGTGSTGNQNPGSPRPGSTGTWNPGSSERGSAGHWTSESSVSGSTGQWHSESGSFRPDSPGSGNARPNNPDWGTFEEVSGNVSPGTRREYHTEKLVTSKGDKELRTGKEKVTSGSTTTTRRSCSKTVTKTVIGPDGHKEVTKEVVTSEDGSDCPEAMDLGTLSGIGTLDGFRHRHPDEAAFFDTASTGKTFPGFFSPMLGEFVSETESRGSESGIFTNTKESSSHHPGIAEFPSRGKSSSYSKQFTSSTSYNRGDSTFESKSYKMADEAGSEADHEGTHSTKRGHAKSRPVRDCDDVLQTHPSGTQSGIFNIKLPGSSKIFSVYCDQETSLGGWLLIQQRMDGSLNFNRTWQDYKRGFGSLNDEGEGEFWLGNDYLHLLTQRGSVLRVELEDWAGNEAYAEYHFRVGSEAEGYALQVSSYEGTAGDALIEGSVEEGAEYTSHNNMQFSTFDRDADQWEENCAEVYGGGWWYNNCQAANLNGIYYPGGSYDPRNNSPYEIENGVVWVSFRGADYSLRAVRMKIRPLVTQ
## 3:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                MVHLTPEEKSAVTALWGKVNVDEVGGEALGRLLVVYPWTQRFFESFGDLSTPDAVMGNPKVKAHGKKVLGAFSDGLAHLDNLKGTFATLSELHCDKLHVDPENFRLLGNVLVCVLAHHFGKEFTPPVQAAYQKVVAGVANALAHKYH
## 4:                                                                                                                                                                                                                                                                                                                                                                                                                                                            MERMLPLLALGLLAAGFCPAVLCHPNSPLDEENLTQENQDRGTHVDLGLASANVDFAFSLYKQLVLKAPDKNVIFSPLSISTALAFLSLGAHNTTLTEILKGLKFNLTETSEAEIHQSFQHLLRTLNQSSDELQLSMGNAMFVKEQLSLLDRFTEDAKRLYGSEAFATDFQDSAAAKKLINDYVKNGTRGKITDLIKDLDSQTMMVLVNYIFFKAKWEMPFDPQDTHQSRFYLSKKKWVMVPMMSLHHLTIPYFRDEELSCTVVELKYTGNASALFILPDQDKMEEVEAMLLPETLKRWRDSLEFREIGELYLPKFSISRDYNLNDILLQLGIEEAFTSKADLSGITGARNLAVSQVVHKAVLDVFEEGTEASAATAVKITLLSALVETRTIVRFNRPFLMIIVPTDTQNIFFMSKVTNPKQA
## 5:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         MKLLTGLVFCSLVLGVSSRSFFSFLGEAFDGARDMWRAYSDMREANYIGSDKYFHARGNYDAAKRGPGGAWAAEVITDARENIQRFFGHGAEDSLADQAANEWGRSGKDPNHFRPAGLPEKY
## 6:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         MKLLTGLVFCSLVLGVSSRSFFSFLGEAFDGARDMWRAYSDMREANYIGSDKYFHARGNYDAAKRGPGGAWAAEVITDARENIQRFFGHGAEDSLADQAANEWGRSGKDPNHFRPAGLPEKY
##    Substrate length       Peptide Start position End position
## 1:              866 FEEVSGNVSPGTR            413          425
## 2:              866      FVSETESR            540          547
## 3:              147       LLVVYPW             32           38
## 4:              423       ITLLSAL            380          386
## 5:              122 FFSFLGEAFDGAR             21           33
## 6:              122    EANYIGSDKY             44           53
# 2. Show which known proteases may have cleaved this protein:

proteases(res) |> head()
##                        Protease name Protease (Uniprot) Protease status
## 1:                       cathepsin D             P07339        reviewed
## 2:                       cathepsin B             P07858        reviewed
## 3:             legumain, animal-type             Q99538        reviewed
## 4:           procollagen C-peptidase             P13497        reviewed
## 5: vertebrate tolloid-like 1 protein             O43897        reviewed
## 6:                       cathepsin D             V9HWI3      unreviewed
##    Protease (MEROPS)                                           Protease URL
## 1:           A01.009 https://www.ebi.ac.uk/merops/cgi-bin/pepsum?id=A01.009
## 2:           C01.060 https://www.ebi.ac.uk/merops/cgi-bin/pepsum?id=C01.060
## 3:           C13.004 https://www.ebi.ac.uk/merops/cgi-bin/pepsum?id=C13.004
## 4:           M12.005 https://www.ebi.ac.uk/merops/cgi-bin/pepsum?id=M12.005
## 5:           M12.016 https://www.ebi.ac.uk/merops/cgi-bin/pepsum?id=M12.016
## 6:           A01.009 https://www.ebi.ac.uk/merops/cgi-bin/pepsum?id=A01.009
# 3. Display details of associated proteolytic events:

cleavages(res) |> head()
##    Substrate (Uniprot)       Peptide Protease (Uniprot) Protease status
## 1:              P02671 FEEVSGNVSPGTR             P07339        reviewed
## 2:              P02671      FVSETESR             P07339        reviewed
## 3:              P68871       LLVVYPW             P07339        reviewed
## 4:              P01011       ITLLSAL             P07339        reviewed
## 5:              P0DJI8 FFSFLGEAFDGAR             P07858        reviewed
## 6:              P0DJI8    EANYIGSDKY             P07858        reviewed
##    Cleaved residue Cleaved terminus Cleavage type
## 1:               F                N physiological
## 2:               F                N physiological
## 3:               L                N  pathological
## 4:               L                C physiological
## 5:               F                N physiological
## 6:               E                N physiological
# We can find out what proportion of matching cleaving events by reviewed
# proteases occur at N- versus C-terminus

cl <- cleavages(res)[`Protease status` == "reviewed"]

cl$`Cleaved terminus` |> table() |> prop.table() |> round(digits = 2)
## 
##   C   N 
## 0.3 0.7
# And inspect the distribution of cleaved amino acids

cl$`Cleaved residue` |> table() |> barplot(cex.names = 2)

# Find which protease is involved in the greatest number of cleaving events

cl[!duplicated(Peptide), .(count = .N), by = `Protease (Uniprot)`]
##    Protease (Uniprot) count
## 1:             P07339     4
## 2:             P07858     2
## 3:             Q99538     1
## 4:             P13497     1
# If start- and end-positions are specified, the function will not attempt
# to automatically look up sequence data for the specified protein/peptide.

cl_by_pos <- findProtease(
    protein = "P02671",
    peptide = "FEEVSGNVSPGTR",
    start_pos = 413,
    end_pos = 425
)

# However, this means sequence details for substrates is not available.

substrates(cl_by_pos)
##            Substrate name Substrate (Uniprot) Substrate sequence
## 1: fibrinogen alpha chain              P02671                 NA
##    Substrate length       Peptide Start position End position
## 1:               NA FEEVSGNVSPGTR            413          425

3.4 Look up a protease on MEROPS

We may want to read up on the details of an entry directly in MEROPS. The function browseProtease takes a UniProt or MEROPS ID and opens the MEROPS summary page which corresponds to that ID in a web browser.

browseProtease("P07339", keytype = "UniprotID") # (opens web browser)

4 Additional examples

4.1 Plot cleavages as a protein-protein interaction network.

Here we visualize the cleaving events of two substrates as a protein-protein interaction network between substrates (red) and associated proteases (blue).

library(igraph)
library(data.table)
# Make a vector of substrates we want to investigate
proteins <- c('P01011','P02671')
# Look up known proteases which cleave these substrates, and associated data
# annotated to the cleaving events
res <- searchSubstrate(protein = proteins, summarize = FALSE)
# Filter to keep proteases with Uniprot status "reviewed"
res <- res[`Protease status` == "reviewed"]
# To create a network, we only need two columns of interactors
# i.e. the proteases with cleaving action on the substrates
res <- res[, c("Protease (Uniprot)", "Substrate (Uniprot)", "Cleavage type")]
# Construct the DAG
g <- igraph::graph_from_data_frame(res,
                                   directed = TRUE,
                                   vertices = unique(
                                       c(res$`Protease (Uniprot)`,
                                         res$`Substrate (Uniprot)`)))
# This will allow us to return to a layout we like
# (and where the legend fits nicely)
set.seed(104)
plot(g,
     vertex.label.family = "Helvetica",
     vertex.size = 14,
     vertex.color = ifelse(V(g)$name %in% res$`Protease (Uniprot)`,
                           "#377EB8", "#E41A1C"),
     vertex.label.cex = 1,
     vertex.label.color = "white",
     edge.arrow.size = .6,
     edge.color =  ifelse(E(g)$`Cleavage type` == "physiological",
                          "#01665E", "#8E0152"),
     layout = igraph::layout.davidson.harel)
legend(title = "Nodes", cex = 2, horiz = FALSE,
       title.adj = 0.0, inset = c(0.0, 0.2),
       "bottomleft", bty = "n",
       legend = c("Protease", "Substrate"),
       fill = c("#377EB8", "#E41A1C"), border = NA)
legend(title = "Edges", cex = 2, horiz = FALSE,
       title.adj = 0.0, inset = c(0.0, 0.0),
       "bottomleft", bty = "n",
       legend = c("Physiological", "Non-physiological"),
       fill = c("#01665E", "#8E0152"), border = NA)