The Nucleocapsid (N) Proteins of Different Human Coronaviruses Demonstrate a Variable Capacity to Induce the Formation of Cytoplasmic Condensates

To date, seven human coronaviruses (HCoVs) have been identified. Four of these viruses typically manifest as a mild respiratory disease, whereas the remaining three can cause severe conditions that often result in death. The reasons for these differences remain poorly understood, but they may be related to the properties of individual viral proteins. The nucleocapsid (N) protein plays a crucial role in the packaging of viral genomic RNA and the modification of host cells during infection, in part due to its capacity to form dynamic biological condensates via liquid-liquid phase separation (LLPS). In this study, we investigated the capacity of N proteins derived from all HCoVs to form condensates when transiently expressed in cultured human cells. Some of the transfected cells were observed to contain cytoplasmic granules in which most of the N proteins were accumulated. Notably, the N proteins of SARS-CoV and SARS-CoV-2 showed a significantly reduced tendency to form cytoplasmic condensates. The condensate formation was not a consequence of overexpression of N proteins, as is typical for LLPS-inducing proteins. These condensates contained components of stress granules (SGs), indicating that the expression of N proteins caused the formation of SGs, which integrate N proteins. Thus, the N proteins of two closely related viruses, SARS-CoV and SARS-CoV-2, have the capacity to antagonize SG induction and/or assembly, in contrast to all other known HCoVs.