Phosphorylation is a critically important modification. Because phospho-peptides are rare and notoriously difficult to detect by MS, good phospho -proteome coverage requires specific enrichment steps.
Phosphorylation is the most studied, and probably the most important, post-translational protein modification. Yet at the same time phospho-peptides remain elusive targets for mass-spectrometry analysis due to their relative rarity and their intrinsic chemical properties.
Experiments that require good coverage of the sample’s phospho-proteome benefit greatly from adding a selective phospho-enrichment step. The enriched sample can then be analysed as a separate fraction in addition to the flow-through or the un-enriched sample. Currently the most-frequently used phospho-enrichment material is Titanium-IMAC.
Alternatively, some fractionation methods selectively enrich for phospho-peptides in specific fractions. HILIC (Hydrophilic Interaction LIquid Chromatography) for instance enriches for hydrophilic PTMs (phosphorylation and hydroxylation) in the latter fractions of the gradient.
Phospho-enrichment can be performed on SILAC- or isobarically (TMT and iTRAQ)- labelled peptides.