Giant KASH proteins and ribosomes synergistically establish cytoplasmic biophysical properties in vivo

Understanding how cells control their biophysical properties during development remains a fundamental challenge. While cytoplasmic macromolecular crowding affects multiple cellular processes in single cells, its regulation in living animals remains poorly understood. Using genetically encoded multimeric nanoparticles for in vivo rheology, we discovered that C. elegans tissues maintain distinct cytoplasmic biophysical properties that differ from those observed across diverse systems, including bacteria, yeast species, and cultured mammalian cells. We identified two conserved mechanisms controlling cytoplasmic macromolecular diffusion: ribosome concentration, a known regulator of cytoplasmic crowding, works in concert with a previously unknown function for the giant KASH protein ANC-1 scaffolding the endoplasmic reticulum. These findings reveal mechanisms by which tissues establish and maintain distinct cytoplasmic biophysical properties, with implications for understanding cellular organization across species. One-Sentence SummaryLiving tissues maintain unique intracellular biophysical properties under the control of cytoplasmic constraints and crowding.