Endosomal maturation leads to nucleocapsid conformation change in seasonal coronaviruses

As seen in the coronavirus pandemic, the social impact of the spread of emerging and re-emerging viral infections is significant. Whilst seasonal human coronaviruses with common cold symptoms have long been identified, there are only few studies on cellular entry pathway into host cells. Human coronavirus (HCoV)-229E, which has a positive single-stranded RNA genome, binds to the host cell surface receptor Aminopeptidase N and is taken up into the cell by endocytosis. The viral genome is then released from the endosome into the cytoplasm via uncoating step, the molecular mechanism of which remains unclear. In the uncoating of influenza A viruses, it is known that after membrane fusion in matured endosomes, unanchored ubiquitin chains present in the viral particle are exposed to the cytoplasmic side, which recruits the host factor histone deacetylase 6 (HDAC6) and promotes M1 uncoating. In this study, purified virus particles were used to investigate the involvement of ubiquitin in the uncoating of coronaviruses, and their properties were determined. The results show that there are few unanchored ubiquitin chains in coronavirus particles, suggesting that they use ubiquitin-independent uncoating different from influenza A virus. More interestingly, acidification within the endosome leads to nucleocapsid condensate formation in vitro. From these results, we would like to propose a new model that coronaviruses with long single-stranded RNA genomes efficiently uncoat their genomes by forming condensates during uncoating, thereby evading the innate immune response from the host. Understanding these molecular mechanisms is expected to lead to drug development.