From birth to degradation, RNA is bound to proteins. When the abundance of specific proteins increases, RNA is sequestered in large assemblies or granules. We investigated the physico-chemical principles promoting formation of granules and observed toxicity when the assemblies are formed in non-physiologial conditions. Indeed, trapping molecules in granules prevents RNA from translation, which perturbes cell functions and impairs general homeostasis. Our work sheds light on the molecular mechanisms behind processes that lead to devastating human diseases.