Four structural proteins are important for coronavirus infectivity: the integral membrane protein M adapts a region of membrane for virus assembly and captures other structural proteins at the budding site, the N protein chaperones and protects the viral RNA genome, spikes consisting of three copies of the S glycoprotein promote receptor-binding and membrane fusion, and the small membrane protein E is present in sub-stoichiometric amounts and acts as an enhancer of budding (Hogue and Machamer, 2007) .