Severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in 2002 as a 
highly transmissible pathogenic human betacoronavirus. The viral spike 
glycoprotein (S) utilizes angiotensin-converting enzyme 2 (ACE2) as a host 
protein receptor and mediates fusion of the viral and host membranes, making S 
essential to viral entry into host cells and host species tropism. As SARS-CoV 
enters host cells, the viral S is believed to undergo a number of conformational 
transitions as it is cleaved by host proteases and binds to host receptors. We 
recently developed stabilizing mutations for coronavirus spikes that prevent the 
transition from the pre-fusion to post-fusion states. Here, we present cryo-EM 
analyses of a stabilized trimeric SARS-CoV S, as well as the trypsin-cleaved, 
stabilized S, and its interactions with ACE2. Neither binding to ACE2 nor 
cleavage by trypsin at the S1/S2 cleavage site impart large conformational 
changes within stabilized SARS-CoV S or expose the secondary cleavage site, S2'.