Eimembraanisista

Eimembraanisista, or non-membrane-bound structures, refers to cellular components that are not enclosed by a lipid bilayer. In Finnish scientific language the term eimembraaninen is used to describe such structures, which contrast with membranous organelles like the nucleus, mitochondria, or endoplasmic reticulum.

These structures are typically dynamic and form through multivalent interactions among proteins and nucleic acids. Many eimembraanisista assemblies arise by phase separation, creating condensates that concentrate specific molecules without a surrounding membrane. They can rapidly assemble and disassemble in response to cellular conditions and stress, and theircomposition often includes intrinsically disordered regions of proteins.

Common examples include the nucleolus, which participates in ribosome biogenesis within the nucleus; ribosomes themselves as large ribonucleoprotein complexes; proteasomes involved in protein degradation; spliceosomes that process RNA transcripts; and cytoplasmic bodies such as stress granules and P-bodies. The centrosome and various cytoskeletal complexes are also considered non-membranous elements of cellular architecture. Together these components support gene expression, RNA processing, protein turnover, and cellular signaling without being bounded by a membrane.

The study of eimembraanisista structures is central to understanding intracellular organization and dynamics. Their formation, regulation, and misregulation are linked to diverse biological processes and diseases, including neurodegeneration and cancer. Research in this area often focuses on how phase separation and molecular interactions drive assembly, maintenance, and disassembly of these membraneless entities.

See also: membranous organelles; phase separation; nuclear bodies; stress granules; P-bodies.