multisubunit

Multisubunit refers to macromolecular complexes that are composed of two or more distinct subunits that come together to form a functional unit. Subunits may be identical, forming homomeric assemblies, or different, forming heteromeric assemblies. Multisubunit organization is common among enzymes, molecular machines, receptors, and structural complexes, enabling regulation, specialization, and combinatorial assembly.

Examples include the proteasome, which contains a core particle formed by α and β subunits and regulatory caps; the ribosome, consisting of large and small subunits made of rRNA and ribosomal proteins; ATP synthase, with rotary rotor and stator subunits; RNA polymerase II in eukaryotes, a holoenzyme built from many different subunits; and photosystem II, a multi-subunit complex in chloroplasts and cyanobacteria.

Protein assembly is often co-translational or assisted by chaperones. Subunit stoichiometry is tightly controlled and interfaces between subunits define the overall architecture. Allosteric interactions among subunits allow cooperative regulation of activity, assembly, or substrate access.

Evolutionarily, multisubunit complexes can arise through gene duplication and diversification; modular design supports functional innovation and robust performance. Defects in subunit composition or assembly can impair function and are linked to diseases in humans and other organisms.

Research methods include genetics, proteomics to identify subunit composition, and structural approaches such as cryo-electron microscopy and X-ray crystallography to determine arrangement and interfaces.