Nonmembranebound

Nonmembranebound describes cellular structures that lack a surrounding lipid bilayer. Also called membraneless organelles, these assemblies reside in the cytoplasm and nucleus and form through multivalent interactions among RNA and proteins, particularly RNA-binding proteins with intrinsically disordered regions. A principal organizing principle is liquid-liquid phase separation, which yields condensates that can concentrate specific factors and exclude others. These structures are dynamic and reversible, frequently assembling rapidly in response to stress, metabolic changes, or transcriptional activity, and dissolving when conditions normalize.

Representative examples include the nucleolus, stress granules, P-bodies, Cajal bodies, and nuclear speckles. The nucleolus functions in ribosomal RNA synthesis and ribosome assembly; stress granules and P-bodies participate in mRNA storage, triage, and decay; Cajal bodies and nuclear speckles are involved in RNA processing and splicing factor organization. As such, nonmembranebound structures provide spatial organization for biochemical reactions without a surrounding membrane, enabling rapid, reversible regulation of RNA metabolism and gene expression.

Functional significance: by operating through phase-separated condensates, they allow cells to concentrate specific components, modulate reaction rates, and respond quickly to changing conditions. Their physical properties render them sensitive to cellular cues and post-translational modifications. Dysregulation of membraneless organelles has been linked to aging and neurodegenerative diseases, where aberrant phase transitions can promote pathological aggregations.

Research and terminology: fields refer to these structures as membraneless organelles or nonmembrane-bound assemblies. Techniques such as fluorescence microscopy and fluorescence recovery after photobleaching reveal dynamic, liquid-like behavior, while in vitro reconstitution demonstrates phase separation with purified proteins and RNAs.