ABSTRACT Viruses need to be activated to infect cells. Activation in this context refers to a virus[unreadable] competence to uncoat and deposit genomes into cells, and is achieved by virus binding with cell receptors, virus acidification, reduction and / or virus proteolysis. Coronaviruses are enveloped RNA viruses causing respiratory diseases. Coronaviruses use a two-step activation strategy involving receptor binding followed by specific proteolysis. While a variety of cellular proteases can activate at this second step in the laboratory, it is not known whether a smaller subset of proteases operate normally in nature. Our central hypothesis in that the proteases relevant to coronavirus activation are those that are naturally (endogenously) linked to the primary virus receptors. We have evidence for a linkage between a transmembrane protease and a human coronavirus receptor and our proposal extends from these findings. Our first aim addresses the possibility that transmembrane proteases activate some coronavirus strains more readily than others, perhaps even before they bind receptors, and asks whether hyper-activation is characteristic of the most pathogenic virus strains. In this aim, we also propose to identify the precise time at which proteases cleave virus spikes. Our results will relate activation to virus virulence and will also give clearer pictures of activation processes. Our second aim is to evaluate connections between receptors and proteases in the more natural context of human lung-like cells and to expand analysis of receptor-protease complexes to the full range of known coronavirus receptors. In this aim, we also propose experiments to separate receptors from proteases and if possible, to find out whether such separation makes cells resistant to infection. Our results will potentially define virus-susceptible cells as those with intact receptor-protease complexes. Our third aim addresses the hypothesis that integrins operate as coreceptors for some coronaviruses. Our central hypothesis oversees this aim. The perspective is that integrins operate similarly to the known primary receptors, transporting bound viruses into regions with abundant activating proteases. This impact of this proposal will be to expand research in virus entry fields beyond studies of virusreceptor interactions and into studies of larger molecular complexes that include virus-activating proteases. This value and health relevance of this proposal will be to reveal new antiviral targets available during the virus entry process.