SARS coronavirus
Alternatively, the virus can enter the host cell directly by proteolytic cleavage of the receptor-attached spike protein by the host's TMPRSS2 or TMPRSS11D serine proteases at the cell surface. In the SARS coronavirus, the activation of the C-terminal part of the spike protein triggers the fusion of the viral envelope with the host cell membrane by inducing conformational changes which are not fully understood.