Interaction directly demonstrated at the atomic level by X-ray crystallography. Also used for NMR or Electron Microscopy (EM) structures. If there is no obvious bait-hit directionality to the interaction involving 3 or more proteins, then the co-crystallized proteins should be listed as a complex.

An interaction is detected between two proteins using chemically reactive or photo-activatable cross-linking reagents that covalently link amino acids in close proximity, followed by mass spectrometry analysis to identify the linked peptides (reviewed in PMID 37406423, 37104977). Experiments may be carried with live cells or cell lysates in which all proteins are expressed at endogenous levels (e.g. PMID 34349018, 35235311) or with recombinant proteins (e.g., PMID 28537071).

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.