The trimeric SARS coronavirus (SARS-CoV) surface spike (S) glycoprotein 
consisting of three S1-S2 heterodimers binds the cellular receptor 
angiotensin-converting enzyme 2 (ACE2) and mediates fusion of the viral and 
cellular membranes through a pre- to postfusion conformation transition. Here, 
we report the structure of the SARS-CoV S glycoprotein in complex with its host 
cell receptor ACE2 revealed by cryo-electron microscopy (cryo-EM). The complex 
structure shows that only one receptor-binding domain of the trimeric S 
glycoprotein binds ACE2 and adopts a protruding "up" conformation. In addition, 
we studied the structures of the SARS-CoV S glycoprotein and its complexes with 
ACE2 in different in vitro conditions, which may mimic different conformational 
states of the S glycoprotein during virus entry. Disassociation of the S1-ACE2 
complex from some of the prefusion spikes was observed and characterized. We 
also characterized the rosette-like structures of the clustered SARS-CoV S2 
trimers in the postfusion state observed on electron micrographs. Structural 
comparisons suggested that the SARS-CoV S glycoprotein retains a prefusion 
architecture after trypsin cleavage into the S1 and S2 subunits and acidic pH 
treatment. However, binding to the receptor opens up the receptor-binding domain 
of S1, which could promote the release of the S1-ACE2 complex and S1 monomers 
from the prefusion spike and trigger the pre- to postfusion conformational 
transition.