Quaternary structure

Definition
noun
A structural level wherein several proteins (or polypeptide subunits) interact through non-covalent bonds to form one functional protein complex.
Supplement
In proteins, there are four recognized levels of protein structure: primary structure, secondary structure, tertiary structure, and quaternary structure.
A quaternary structure of a protein refers to the assembly of multiple folded protein molecules in a multi-subunit complex.
See also: protein structure