What is a macromolecular complex?

A macromolecular complex is a stable set of (two or more) interacting protein molecules which:

1. Can be co-purified, and
2. Have been shown to exist as a functional unit in vivo

Non-protein molecules (e.g. small molecules, nucleic acids) may also be present in the complex if they are an integral part of the complex.

The following are not considered to be stable complexes:

• Two or more proteins associated in a pulldown / coimmunoprecipitation with no functional link
• Genetic interaction evidences
• Transient interactions such as enzyme-substrate or receptor-ligand complexes that have a 1:1 relationship

There are some exceptions to these rules, as follows:

• An enzyme or receptor is a complex in its own right
• An enzyme or receptor is an obligate complex that requires substrate/ligand binding for its functional assembly

For example:

• Maltose transporter (the enzyme) requires maltose (the substrate) binding for formation of the functional enzyme (CPX-1932)
• PDGF receptors: the PDGF ligands are obligate dimers and receptors require ligand binding for dimerisation of the two receptor chains forming an obligate tetramer (e.g. CPX-2885)