Higher-Order Protein Organization: Extending From Four to Nine Levels

The classical protein structure hierarchy-primary, secondary, tertiary, and quaternary-defines oligomeric complexes as the upper limit of molecular organization. However, accumulating evidence reveals that metabolic enzymes, including CTP synthase (CTPS) and Δ¹-pyrroline-5-carboxylate synthetase (P5CS), assemble into long-lived, filamentous structures in vivo that extend beyond this traditional framework. Here, I propose a nine-level structural continuum spanning from the primary amino acid sequence to integrated cytoophidium networks. This taxonomy employs Latin numerals to categorise these structured nested architectures, thereby distinguishing them from transient quinary interactions and the labile nature of biomolecular associations. Terms such as quinternary (metabolic filaments), senary (metabolic cables), septenary (metabolic meshes), octonary (cytoophidia), and nonary (cytoophidium complexes) replace ambiguous descriptors like 'metabolic foci' or 'aggregates.' Crucially, this framework establishes higher-order assemblies as an emerging paradigm of cellular compartmentation. Given the widespread conservation of these structures across the three domains of life, this taxonomy provides a conceptual foundation for understanding how cells construct complexity beyond membrane-bound systems.