Abstracts
Fritz Lipmann Lecture:
Multi-Scale Structure of Chromatin Condensates
Michael Rosen
Department of Biophysics, University of Texas Southwestern Medical Center, Howard Hughes Medical Institute, Dallas, Texas (USA)
Biomolecular condensates organize cellular biochemistry in a wide array of processes including signaling, RNA metabolism and gene regulation. Condensates are also defective in diseases including neurodegeneration and cancer. Condensate functions are believed to be strongly modulated by the composition and material properties of their internal fluids, which in turn derive from the structure and higher-order organization of resident macromolecular scaffolds. Yet visualization of scaffold molecules is complicated by the dynamic and heterogeneous nature of the condensate interior, making structure-function relationships mysterious for these meso-scale systems.
We have shown that polynucleosome arrays, which mimic cellular chromatin, can phase separate under physiologic salt conditions. Since individual nucleosomes can be directly imaged using cryo-electron tomography (cryoET), phase separated chromatin is ideal to examine relationships between condensate structure, material properties, and function. I will describe a pipeline spanning sample preparation, data acquisition, and image analysis that has enabled us to accurately identify the spatial positions and contacts of virtually all nucleosomes in frozen slabs of chromatin condensates. These data have enabled us to rationalize the structural changes that drive chromatin phase separation, and the differences in material properties of condensates produced from different chromatin molecules. Notably, the internal structure of one class of chromatin condensates is highly similar to that observed in dense nucleosome assemblies in cells, suggesting the condensates may mimic certain aspects of cellular chromatin. Our procedures should be generally applicable to structural studies of large molecular species contained within biochemically reconstituted condensates, and perhaps also within native condensates in cells. Our study illustrates how meso-scale structure can inform on biological function.