PSM: Handling and Managing Peptide Spectrum Matches

adjacencyMatrix

R Documentation

Convert to/from an adjacency matrix.

Description

There are two ways that peptide/protein matches are commonly stored: either as a vector or an adjacency matrix. The functions described below convert between these two format.

Usage

makeAdjacencyMatrix(
  x,
  split = ";",
  peptide = psmVariables(x)["peptide"],
  protein = psmVariables(x)["protein"],
  score = psmVariables(x)["score"],
  binary = FALSE
)

makePeptideProteinVector(m, collapse = ";")

plotAdjacencyMatrix(
  m,
  protColors = 0,
  pepColors = NULL,
  layout = igraph::layout_nicely
)

Arguments

`x`	Either an instance of class `PSM` or a `character`. See example below for details.
`split`	`character(1)` defining how to split the string of protein identifiers (using `strsplit()`). Default is ";". If `NULL`, splitting is ignored.
`peptide`	`character(1)` indicating the name of the variable that defines peptides in the `PSM` object. Default is the `peptide` PSM variable as defined in `psmVariables()`.
`protein`	`character(1)` indicating the name of the variable that defines proteins in the `PSM` object. Default is the `peptide` PSM variable as defined in `psmVariables()`.
`score`	`character(1)` indicating the name of the variable that defines PSM scores in the `PSM` object. Default is the `score` PSM variable as defined in `psmVariables()`. Ignored when `NA` (which is the default value unless set by the user when constructing the `PSM` object).
`binary`	`logical(1)` indicates if the adjacency matrix should be strictly binary. In such a case, PSMs matching the same peptide but from different precursors (for example charge 2 and 3) or carrying different PTMs, are counted only once. Default if `FALSE`. This also overrides any `score` that would be set.
`m`	A peptide-by-protein adjacency matrix.
`collapse`	`character(1)` indicating how to collapse protein names for shared peptides. Default is `";"`.
`protColors`	Either a `numeric(1)` or a named `character()` of colour names. The numeric value indicates the protein colouring level to use. If 0 (default), all protein nodes are labelled in steelblue. For values > 0, approximate string distances (see `adist()`) between protein names are calculated and nodes of proteins that have names that differ will be coloured differently, with higher values leading to more colours. While no maximum to this value is defined in the code, it shouldn't be higher than the number of proteins. If a character is used, it should be a character of colour names named by protein identifiers. That vector should provide colours for at least all proteins in the adjacency matrix `m`, but more protein could be named. The latter is useful when generating a colour vector for all proteins in a dataset and use it for different adjacency matrix visualisations.
`pepColors`	Either `NULL` (default) for no peptide colouring (white nodes) or a named `character()` of colour names. It should be a character of colour names named by peptide identifiers. That vector should provide colours for at least all peptides in the adjacency matrix `m`, but more peptides could be named. The latter is useful when generating a colour vector for all peptides in a dataset and use it for different adjacency matrix visualisations.
`layout`	A graph layout, as defined in the `ipgraph` package. Default is `layout_as_bipartite()`.

Details

The makeAdjacencyMatrix() function creates a peptide-by-protein adjacency matrix from a character or an instance of class PSM().

The character is formatted as x <- c("ProtA", "ProtB", "ProtA;ProtB", ...), as commonly encoutered in proteomics data spreadsheets. It defines that the first peptide is mapped to protein "ProtA", the second one to protein "ProtB", the third one to "ProtA" and "ProtB", and so on. The resulting matrix contain length(x) rows an as many columns as there are unique protein idenifiers in x. The columns are named after the protein idenifiers and the peptide/protein vector namesa are used to name to matrix rows (even if these aren't unique).

The makePeptideProteinVector() function does the opposite operation, taking an adjacency matrix as input and retruning a peptide/protein vector. The matrix colnames are used to populate the vector and the matrix rownames are used to name the vector elements.

Note that when creating an adjacency matrix from a PSM object, the matrix is not necessarily binary, as multiple PSMs can match the same peptide (sequence), such as for example precursors with different charge states. A binary matrix can either be generated with the binary argument (setting all non-0 values to 1) or by reducing the PSM object accordingly (see example below).

It is also possible to generate adjacency matrices populated with identification scores or probabilites by setting the "score" PSM variable upon construction of the PSM object (see PSM() for details). In case multiple PSMs occur, their respective scores get summed.

The plotAdjacencyMatrix() function is useful to visualise small adjacency matrices, such as those representing protein groups modelled as connected components, as described and illustrated in ConnectedComponents(). The function generates a graph modelling the relation between proteins (represented as squares) and peptides (represented as circes), which can further be coloured (see the protColors and pepColors arguments). The function invisibly returns the graph igraph object for additional tuning and/or interactive visualisation using, for example igraph::tkplot().

There exists some important differences in the creation of an adjacency matrix from a PSM object or a vector, other than the input variable itself:

In a PSM object, each row (PSM) refers to an individual proteins; rows/PSMs never refer to a protein group. There is thus no need for a split argument, which is used exclusively when creating a matrix from a character.
Conversely, when using protein vectors, such as those illustrated in the example below or retrieved from tabular quantitative proteomics data, each row/peptide is expected to refer to protein groups and individual proteins (groups of size 1). These have to be split accordingly.

Value

A peptide-by-protein sparce adjacency matrix (or class dgCMatrix as defined in the Matrix package) or peptide/protein vector.

Author(s)

Laurent Gatto

Examples


## -----------------------
## From a character
## -----------------------

## Protein vector without names
prots <- c("ProtA", "ProtB", "ProtA;ProtB")
makeAdjacencyMatrix(prots)

## Named protein vector
names(prots) <- c("pep1", "pep2", "pep3")
prots
m <- makeAdjacencyMatrix(prots)
m

## Back to vector
vec <- makePeptideProteinVector(m)
vec
identical(prots, vec)

## ----------------------------
## PSM object from a data.frame
## ----------------------------

psmdf <- data.frame(psm = paste0("psm", 1:10),
                    peptide = paste0("pep", c(1, 1, 2, 2, 3, 4, 6, 7, 8, 8)),
                    protein = paste0("Prot", LETTERS[c(1, 1, 2, 2, 3, 4, 3, 5, 6, 6)]))
psmdf
psm <- PSM(psmdf, peptide = "peptide", protein = "protein")
psm
makeAdjacencyMatrix(psm)

## Reduce PSM object to peptides
rpsm <- reducePSMs(psm, k = psm$peptide)
rpsm
makeAdjacencyMatrix(rpsm)

## Or set binary to TRUE
makeAdjacencyMatrix(psm, binary = TRUE)

## ----------------------------
## PSM object from an mzid file
## ----------------------------

f <- msdata::ident(full.names = TRUE, pattern = "TMT")
psm <- PSM(f) |>
       filterPsmDecoy() |>
       filterPsmRank()
psm
adj <- makeAdjacencyMatrix(psm)
dim(adj)
adj[1:10, 1:4]

## Binary adjacency matrix
adj <- makeAdjacencyMatrix(psm, binary = TRUE)
adj[1:10, 1:4]

## Peptides with rowSums > 1 match multiple proteins.
## Use filterPsmShared() to filter these out.
table(Matrix::rowSums(adj))

## -----------------------------------------------
## Binary, non-binary and score adjacency matrices
## -----------------------------------------------

## -------------------------------------
## Case 1: no scores, 1 PSM per peptides
psmdf <- data.frame(spectrum = c("sp1", "sp2", "sp3", "sp4", "sp5",
                                 "sp6", "sp7", "sp8", "sp9", "sp10"),
                    sequence = c("NKAVRTYHEQ", "IYNHSQGFCA", "YHWRLPVSEF",
                                 "YEHNGFPLKD", "WAQFDVYNLS", "EDHINCTQWP",
                                 "WSMKVDYEQT", "GWTSKMRYPL", "PMAYIWEKLC",
                                 "HWAEYFNDVT"),
                    protein = c("ProtB", "ProtB", "ProtA", "ProtD", "ProtD",
                                "ProtG", "ProtF", "ProtE", "ProtC", "ProtF"),
                    decoy = rep(FALSE, 10),
                    rank = rep(1, 10),
                    score = c(0.082, 0.310, 0.133, 0.174, 0.944, 0.0261,
                              0.375, 0.741, 0.254, 0.058))
psmdf

psm <- PSM(psmdf, spectrum = "spectrum", peptide = "sequence",
           protein = "protein", decoy = "decoy", rank = "rank")

## binary matrix
makeAdjacencyMatrix(psm)

## Case 2: sp1 and sp11 match the same peptide (NKAVRTYHEQ)
psmdf2 <- rbind(psmdf,
                data.frame(spectrum = "sp11",
                           sequence = psmdf$sequence[1],
                           protein = psmdf$protein[1],
                           decoy = FALSE,
                           rank = 1,
                           score = 0.011))
psmdf2
psm2 <- PSM(psmdf2, spectrum = "spectrum", peptide = "sequence",
            protein = "protein", decoy = "decoy", rank = "rank")

## Now NKAVRTYHEQ/ProtB counts 2 PSMs
makeAdjacencyMatrix(psm2)

## Force a binary matrix
makeAdjacencyMatrix(psm2, binary = TRUE)

## --------------------------------
## Case 3: set the score PSM values
psmVariables(psm) ## no score defined
psm3 <- PSM(psm, spectrum = "spectrum", peptide = "sequence",
            protein = "protein", decoy = "decoy", rank = "rank",
            score = "score")
psmVariables(psm3) ## score defined

## adjacency matrix with scores
makeAdjacencyMatrix(psm3)

## Force a binary matrix
makeAdjacencyMatrix(psm3, binary = TRUE)

## ---------------------------------
## Case 4: scores with multiple PSMs

psm4 <- PSM(psm2, spectrum = "spectrum", peptide = "sequence",
            protein = "protein", decoy = "decoy", rank = "rank",
            score = "score")

## Now NKAVRTYHEQ/ProtB has a summed score of 0.093 computed as
## 0.082 (from sp1) + 0.011 (from sp11)
makeAdjacencyMatrix(psm4)

rformassspectrometry/PSM documentation built on Feb. 2, 2025, 1:52 a.m.