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Mapping translation rates

This is a repository containing the analysis scripts for our comprehensive mapping of translation rates in the HepG2 liver cancer cell line. The underlying data set is a multi-omics data set with data from RNA sequencing, ribosome profiling, and proteomics. The data encompasses data from widely used steady state techniques like RNA sequencing, ribosome profiling, and proteomics. Moreover, we use RNA sequencing of different fractions obtained from polysome profiling to calculate mean ribosome load measures, ribosome profiling time courses after harringtonine treatment to observe ribosome runoff, and pulsed SILAC to measure the rate of protein buildup in HepG2 cells, i.e. we complement the standard (steady-state) techniques with methods that can resolve the time-dimension. While data analysis of the former is quite well-established, we publish the analysis scripts for the less standard methods (polysome profiling, ribosome runoff, pSILAC).

Mean ribosome load from polysome profiling

The script polysome_analysis_model_RL_pool.py performs the analysis of polysome profiling data.

Input data

The script needs three input files and some column names:

  1. a count matrix of RNA seq reads mapped to any exon of a particular gene (tsv format)
  2. a tsv file that coordinates gene IDs with transcript IDs (needs columns named "gene_ID" and "transcript_ID")
  3. a fasta file of the transcriptome (possibly assemble with gffread) to extract the transcript length and if the transcript codes for a protein
  4. specify column names of the fractions together with the corresponding number of ribosomes in the sample_dict dictionary, and the replicate names in the replicates array. The remainder fraction is treated separately and its name should be specified as the heavy_fract.

The names of these input files can be specified in the upper paragraph of the script.

Output data

The produced output is a tsv file containing the frequency of one gene in each fraction, the ribosome load, the fitting error of the ribosome load, and a relative estimate of the reduced chi^2. Relative means its absolute size is not meaningful, but it can be used as a criterion for fit likelihood when comparing genes.

Procedure

Experimentally, an assessment of the total amount of RNA in each fraction is possible by processing the isolated RNA together with a given amount of spikein RNA, that is kept constant among fractions. To this end, the spikein sequences have to be amended to the genome that is used for mapping (ENSEMBL GRCH 38.105 and lexogen SIRV set 3 in our case).

Computationally, we extract mappings to exons with Rsubread for our initial count matrix. Then, the script:

  • extracts transcript lengths and if they are coding for proteins from the transcriptome fasta file
  • computes the frequencies of transcripts (per fraction, ratio of TPMs) from the count matrix
  • computes relative representation of each fraction (per gene) and fits a Poisson distribution (first moment of the Poisson distribution is lambda, i.e. the mean ribosome load)
  • saves the frequencies, lambda, error of lambda, and relative reduced chi^2 into a tsv file

This method is superior to just computing the weighted average of ribosomes per gene, since it is not intrinsically limited to the maximum fraction that you can resolve (10 in our case).

Global translation elongation rates from runoff ribosome profiling

Global translation elongation rates can be determined from metagene-ribosome profiles via the SL method (Ingolia et. al.. 2011. “Ribosome Profiling of Mouse Embryonic Stem Cells Reveals the Complexity and Dynamics of Mammalian Proteomes.” Cell 147 (4)). To this end, the folder runoff_riboseqcontains two python scripts: depletion_curves.py creates a plot of ribosome depletion curves characteristic to harringtonine + cycloheximide treatment, SL_determination_fit.py extracts global elongation speeds from these curves.

Input depletion_curves.py

This script needs two kinds of input files and a couple of parameters:

  1. a .tsv file with the CDS coordinates on the transcriptome, transcript ID and gene ID
  2. a dictionary with time differences between HRT and CHX application, and the corresponding (.sam/.bam) mapping files together with .json files with the p-site offset estimations
  3. the minimum length of the transcripts contributing to the metagene plot (3000nt in Ingolia et. al.)
  4. the binsize (in nts) over which the coverage is averaged (5codons = 15nts in Ingolia et. al.)
  5. the size of the plateau at the end of the profile (in nts, 600 in the original publication)

Procedure depletion_curves.py

After reading the coordinate file, the script reads the mapping files and constructs the riboseq profiles at different time separations between application of harringtonine and cycloheximide. These curves are plotted and saved into a text file

Procedure SL_determination_fit.py

An arbitrary amount of these text files serve as a starting point for the determination of the elongation speed with the SL method. The procedure for each single one of these files is:

  1. the input files are read, the starting locations (where t > 0 curve / t = 0 curve passes 0.5 for the first time from below) are determined
  2. a straight line is fitted through the starting locations at different t > 0
  3. data and fit line are added to an overall plot

Outputs

The joint output of these two scripts are:

  1. text files containing depletion curves from every analyzed time course
  2. .pdf files with plots of these depletion curves (one per time course)
  3. .pdf file with linear fits (one file in total)

Protein synthesis rates from pSILAC

The script differential_protein_synthesis.py performs the analysis of pSILAC data.

Input data

The script needs the following inputs:

  1. a .csv file with (log2-fold) peptide intensities
  2. specify the columns in the time_cols_dict dictionary, together with the time differences (in minutes) between switching from heavy to light amino acids and measurement
  3. the identifier of the replicate to analyze
  4. a .csv file with the protein copy numbers per cell (e.g. Wisniewski et. al.. 2016. “In-Depth Quantitative Analysis and Comparison of the Human Hepatocyte and Hepatoma Cell Line HepG2 Proteomes.” Journal of Proteomics)
  5. specify the column names over which the protein copy number should be averaged in copy_number_columns
  6. a .tsv file that connects ENSEMBL gene IDs to transcript IDs, swissprot and uniprot IDs

Procedure

With this input at hand, we proceed as follows:

  1. only keep the peptides that are labeled as heavy (in our case either by Lys8LFQor Arg10LFQ)
  2. read off the protein id and the peptide sequence from the peptide identifier
  3. count the number of time points which are resolved per peptide
  4. fit a linear curve with negative slope to the intensities of the heavily labeled peptides (positive slope if doing the transition light -> heavy at t=0). Since Peptide intensities themselves are not meaningful, we can project them out using variable projection (O’Leary, Rust 2012: Variable Projection for Nonlinear Least Squares Problems) to make the fit more stable.
  5. average over multiple peptides being a part of a single protein to end up with protein buildup rates
  6. normalize by protein abundance per cell to end up with protein synthesis rates per cell

Output data

The code generates output .tsv files at different stages:

  1. peptide buildup rates (log intensities, preptide ID split up in protein ID and peptide sequence, number of time points going into the fit, resulting rate)
  2. tidied up peptide buildup rates (same as above, removed spurious rates smaller 1e-5)
  3. protein buildup rates: peptide buildup rates averaged over (tidy) peptides corresponding to the same protein (swissprot id, rate, spread over which the different peptide estimates going into the rate range)
  4. protein synthesis rates: buildup rates normalized by protein abundance per cell (swissprot id, HGNC gene symbo, ensembl gene id, protein copy number per cell averaged over replicates, protein synthesis rate, spread of the protein synthesis rate)

License

The code is published under the MIT license.