Evidence for widespread cytoplasmic structuring into mesoscale condensates.

Area of Science:

• Cell Biology
• Biochemistry
• Molecular Biology

Background:

• Eukaryotic cells utilize membrane-bound organelles and membrane-less biomolecular condensates for compartmentalization.
• Biomolecular condensates are typically liquid-like, observed at micrometer scales (~1 µm), and studied via microscopy focusing on individual proteins.
• Their functions include regulating transcription, RNA processing, and signaling, with malfunctions linked to diseases.

Purpose of the Study:

• To investigate the global extent and typical length scales of biomolecular condensates in eukaryotic cytoplasm.
• To challenge the prevailing view of biomolecular condensates forming primarily at micrometer scales.

Main Methods:

• Quantitative proteomics, filtration, size exclusion, and dilution experiments on native Xenopus egg extract.
• Imaging below the diffraction limit using protein permeation into porous substrates with defined pore sizes.

Main Results:

• At least 18% of the proteome is organized into mesoscale biomolecular condensates (~100 nm).
• These mesoscale condensates are stabilized by RNA or gelation.
• Confirmed mesoscale sizes using sub-diffraction limit imaging techniques.

Conclusions:

• Eukaryotic cytoplasm is extensively organized by biomolecular condensates.
• Contrary to previous assumptions, these structures predominantly form at smaller, mesoscale (nanometer) length scales.
• This finding redefines our understanding of cellular organization and the role of biomolecular condensates.