Interferon-gamma regulates liquid-liquid phase separation via the caspase like activity of the proteasome

Liquid-liquid phase separation (LLPS) molecular compensate structures are found primarily in eukaryotic cells. These structures concentrate various macromolecules without an encapsulating membrane. LLPS regulate diverse cellular biological functions and are involved in diseases such as neurodegenerations and cancers. However, these structures are very dynamic, and their ingredients constantly exchange in and out. We investigated the formation and dynamics of both stress granules and proteasome granules LLPS structures. We found that cells treated with interferon-gamma are inefficient in forming these granules. Proteasomes of cells treated with interferon-gamma are very poor in caspase-like (CL) activity, whereas the trypsin-like and chymotrypsin-like activities are unaffected. We established a cell line bearing an enzymatically inactive mutant of the CL subunit to investigate the possibility that CL activity is important for forming the granules. Cells were treated with either arsenite or osmotic stresses to induce granule formation. We found that in CL minus cells, the granules are poorly formed and reach maximal levels significantly below that of wild-type cells. These findings suggest that interferon-gamma regulates liquid-liquid phase separation via downregulating the caspase-like activity of the proteasome.