Analysing protein interactions. In the 'co-precipitation/mass spectrometry' approach used by Gavin et al.1 and Ho et al.2, an 'affinity tag' is first attached to a target protein (the 'bait'; a). b, Bait proteins are systematically precipitated, along with any associated proteins, on an 'affinity column'. c, Purified protein complexes are resolved by one-dimensional SDS-PAGE, a technique that involves running an electric charge through the complexes on a gel, so that proteins become separated according to mass. d, Proteins are excised from the gel, digested with the enzyme trypsin, and analysed by mass spectrometry. Database-search algorithms (bioinformatics) are then used to identify specific proteins from their mass spectra.

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