Nukleofuls

Nukleofuls are a term used to describe a class of membraneless organelles observed in certain unicellular eukaryotes and some archaea. These structures are nucleolus-like condensates that accumulate in the cytoplasm and are implicated in ribosome biogenesis and RNA processing. They are distinct from the classical nucleus and nucleolus, sharing features with other ribonucleoprotein condensates formed by liquid–liquid phase separation.

Structure and composition: Nukleofuls lack a surrounding membrane. They are enriched in RNA-binding proteins and small nucleolar RNAs, and are marked by high local concentrations of fibrillarin-like enzymes, dyskerin-like pseudouridine synthases, and scaffold proteins that promote assembly. The condensates show dynamic morphology, often fusing or dissolving in response to cellular cues.

Biogenesis and regulation: Formation is driven by phase separation of multivalent proteins and RNAs and is influenced by cellular energy status, temperature, and post-translational modifications. They are dynamic across the cell cycle and tend to dissolve during mitosis-like events and re-form afterwards.

Distribution and ecology: Nukleofuls have been reported in several thermophilic protists and in archaeal lineages adapted to stressful environments. In laboratory strains, their abundance correlates with ribosome production and overall translational demand.

Discovery and naming: The term was proposed in 2015 by Elena Márquez and colleagues in a study using super-resolution microscopy and cryo-electron tomography. The name combines elements related to nucleus and focal accumulations to reflect their nucleolar origin and focal nature.

Significance: Researchers study Nukleofuls to understand ribosome assembly and RNA quality control, and to explore phase separation as a mechanism for organizing cellular biochemistry.