Proteomics

proteomics studies the identities, amounts, modifications and associations of proteins in biological material. Measurements depend on extraction, detectability and reference condition; absence of a detected peptide is not proof that its protein is absent.

Proteome-scale assignment of proteins to cellular locations using fractionation profiles, localization labeling and complementary imaging.

Quantitative proteomics measures differences in protein abundance or related protein features across samples. Strategies include label-free quantification and isotope- or tag-based comparisons, often using peptides generated from proteins and analyzed by mass spectrometry.

Peptide signal must be connected to the protein being quantified. Shared peptide sequences, incomplete detection, and changes in modification state can complicate that inference. Absolute abundance and relative change therefore require different calibration and interpretation.

Label-free quantification compares abundance across separately measured samples without introducing a distinguishing isotope or chemical tag. In proteomics, a common approach integrates chromatographic signals of corresponding peptide ions.

Matching retention time and mass helps connect the same peptide across runs, while normalization addresses overall signal differences. A missing peak can reflect limited detection rather than true absence, so zeros and missing measurements should not be conflated.

A tandem mass tag is a chemical label designed so that differently encoded versions have the same overall mass but release distinguishable reporter ions during fragmentation. Labeled peptide samples can therefore be mixed and compared in one mass-spectrometric analysis.

Reporter-ion intensities estimate relative amounts for the pooled channels. Co-isolated peptides can contribute unrelated reporter signal and compress measured ratios, linking instrument isolation selectivity to the accuracy of biological comparisons.

Stable isotope labeling by amino acids in cell culture incorporates isotopically distinct amino acids into proteins during cell growth. Mixing differently labeled samples produces peptide pairs with predictable mass differences, enabling relative quantification by mass spectrometry.

The labels preserve closely related chemistry while changing mass. Interpretation requires adequate incorporation and awareness of amino-acid conversion or growth-related differences. The method compares labeled protein populations; it does not make every observed abundance change a direct change in protein synthesis rate.

Analysis of relatively large protein fragments to preserve linkage between sequence changes or modifications that separate into different peptides in bottom-up proteomics. It generally preserves less complete linkage than intact-chain analysis.

Analysis of intact protein ions and their sequence fragments to characterize proteoforms. Intact mass constrains composition, while informative fragmentation is needed to locate changes and resolve many isomeric forms.

An error in assigning a complete molecular form. Controlling peptide-level identification error does not automatically control errors in intact-form identity or modification linkage.

Analysis of proteins after digestion into smaller peptides. It can provide broad sequence and modification coverage, but digestion commonly loses information about which distant variations occur together in an intact proteoform.

A molecular form of a protein defined by its amino-acid sequence and covalent modifications, including processing. Different conformations and mass-spectrometric charge states do not by themselves define different proteoforms.