Recently, coronaviruses have generated substantial interest from the biomedical community based on the emergence and isolation of a novel and deadly coronavirus infecting humans. This virus was recently named Middle East Respiratory Syndrome (MERS)-CoV. Like other enveloped viruses, coronaviruses access their host cells by membrane fusion, a process mediated by a specific fusion, or "spike" protein on the virion. Such viral fusion proteins are often activated by host cell proteases. Mutations in the virus genome can lead to modifications to the proteolytic cleavage sites(s) on the spike that can subsequently regulate membrane fusion, virus entry and host cell tropism. We have identified changes in the MERS-CoV spike protein cleavage site(s) that distinguish it from its closest relatives, as well as most other coronaviruses. These changes allow cleavage-activation of the spike protein by furin, a broadly expressed protease. Furin-mediated activation is unusual in that it occurs during virus entry. The ability of MERS-CoV to utilize furin in this manner may explain the polytropic nature of the virus and its life cycle as a zoonotic coronavirus. In this application, we will characterize the proteolytic cleavage sites of MERS-CoV spike, in comparison to its closest relatives BatCoV- HKU4 and BatCoV-HKU5, and determine the role of furin-mediated cleavage in the context of the polytropic nature of MERS-CoV. More broadly, our goals are to understand how mutations in the CoV spike cleavage sites modulate protease cleavage and fusion activation. Our work will be useful for public health planning and in testing the feasibility of using furin inhibitors as anti-viral therapeutics for MERS-like CoV infections.