An interaction is inferred when a bait protein is affinity captured from cell extracts by either polyclonal antibody or epitope tag and the associated interaction partner identified by Western blot with a specific polyclonal antibody or second epitope tag. This category is also used if an interacting protein is visualized directly by dye stain or radioactivity. Note that this differs from any co-purification experiment involving affinity capture in that the co-purification experiment involves at least one extra purification step to get rid of potential contaminating proteins.

An interaction is inferred from the biochemical effect of one protein upon another, for example, GTP-GDP exchange activity or phosphorylation of a substrate by a kinase. The bait protein executes the activity on the substrate hit protein. A Modification value is recorded for interactions of this type with the possible values Phosphorylation, Ubiquitination, Sumoylation, Dephosphorylation, Methylation, Prenylation, Acetylation, Deubiquitination, Proteolytic Processing, Glucosylation, Nedd(Rub1)ylation, Deacetylation, No Modification, Demethylation.

An interaction is inferred between proteins in vitro. This can include proteins in recombinant form or proteins isolated directly from cells with recombinant or purified bait. For example, GST pull-down assays where a GST-tagged protein is first isolated and then used to fish interactors from cell lysates are considered reconstituted complexes (e.g. PUBMED: 14657240, Fig. 4A or PUBMED: 14761940, Fig. 5). This can also include gel-shifts, surface plasmon resonance, isothermal titration calorimetry (ITC) and bio-layer interferometry (BLI) experiments. The bait-hit directionality may not be clear for 2 interacting proteins. In these cases the directionality is up to the discretion of the curator.