subunitsubunit

Subunitsubunit is a descriptive term used in structural biology to denote the interface between two subunits within a multimeric protein complex, or a functional module formed by cooperating subunits. It highlights the recurring assembly motifs that stabilize quaternary structure and coordinate activity. The term is not part of formal nomenclature in all fields; it is commonly used interchangeably with subunit interface, subcomplex, or quaternary-structure contact in review contexts.

Subunitsubunit interfaces typically involve a combination of hydrophobic contacts, hydrogen bonds, salt bridges, and sometimes metal coordination. They can be tight and enduring or transient during assembly or regulation, and they often determine the stoichiometry and symmetry of the complex as well as allosteric communication between subunits.

Study of subunitsubunit contacts relies on structural methods such as X-ray crystallography and cryo-electron microscopy, supplemented by cross-linking mass spectrometry, nuclear magnetic resonance, and mutagenesis. Biophysical assays and computational modeling are used to estimate interface strength, dynamics, and the energetic contribution of individual residues.

Understanding subunitsubunit interactions has practical implications for biology and medicine. Disruption or strengthening of these interfaces can modulate enzymatic activity, assembly pathways, or signaling networks, and defects at subunitsubunit interfaces are implicated in various diseases. The concept is widely applicable to enzymes, ribonucleoprotein complexes, membrane channels, and large molecular machines.

See also: protein-protein interaction, oligomerization, protein complex, quaternary structure.