Overview
Targeted proteomics focuses on quantifying a predefined set of proteins with high sensitivity, specificity, and throughput, in contrast to discovery proteomics which aims for broad, untargeted coverage. The most widely adopted technique is selected reaction monitoring (SRM, also called multiple reaction monitoring or MRM) performed on triple quadrupole mass spectrometers. SRM assays target specific peptide-precursor-to-fragment transitions, known as proteotypic peptides, that uniquely represent the protein of interest. This approach delivers quantitative precision comparable to immunoassays while offering higher multiplexing capability and faster assay development.
Methods
Assay development begins with the selection of proteotypic peptides: peptides that are unique to the target protein, fly well in the mass spectrometer, and produce intense fragment ions. For each peptide, several transitions (precursor ion m/z to fragment ion m/z) are monitored across the chromatographic elution period. Internal standards, typically stable isotope-labeled versions of the target peptides, are spiked into the sample at known concentrations to enable absolute quantification. Parallel reaction monitoring (PRM) on Orbitrap instruments offers a related approach with high-resolution full-spectrum acquisition. Data-independent acquisition (DIA) methods such as SWATH-MS bridge the gap between discovery and targeted analysis by recording all fragment ions from all precursors in a systematic fashion.
Practical Protocol
Developing an SRM/MRM assay starts with identifying proteotypic peptides for the target protein. Using spectral libraries from PeptideAtlas or SRMAtlas, the researcher selects peptides that are 7-25 amino acids long, lack missed cleavage sites, avoid methionine and tryptophan (which are prone to oxidation), and produce intense y- or b-ion fragments from mass spectrometry data. For each peptide, 3-5 transitions are selected - the top 3-5 most intense fragment ions from the library spectrum. A heavy isotope-labeled standard peptide is synthesized for each target, incorporating 13C/15N-labeled arginine or lysine to produce a mass shift of 6-10 Da. The heavy standard is spiked into each sample at a known concentration prior to digestion or after digestion depending on the quantification strategy. The triple quadrupole mass spectrometer is configured with Q1 and Q3 transmitting the precursor and fragment m/z values, respectively, while Q2 performs collision-induced fragmentation at optimized collision energies specific to each transition. Scheduled SRM monitors each transition within a 2-3 minute retention time window, maximizing the number of peptides quantified per run. Raw data are processed in Skyline: chromatographic peaks are integrated, retention times are confirmed against the heavy internal standard, and the light-to-heavy peak area ratio is calculated. Absolute concentration is derived from this ratio and the known standard concentration. A real-world example: a multiplexed MRM assay quantifying 50 cancer-associated proteins in plasma was developed for early detection of non-small cell lung cancer, achieving limits of detection below 1 ng/mL for 35 of the 50 targets. The assay was validated across 500 patient samples with coefficients of variation below 15%.
Applications
Targeted proteomics is the method of choice for verifying and validating candidate biomarkers discovered in untargeted studies. It is used extensively in clinical research to quantify panels of proteins linked to specific diseases. The assays complement traditional ELISA measurements, offer an alternative to gel-based approaches and capillary gel electrophoresis, and can be multiplexed to track dozens of proteins in a single run. Integration with HPLC separation and mass spectrometry detection ensures robust quantification, while protein quantification assays provide orthogonal validation of total protein concentration.
