Molecular Advances in Severe Acute Respiratory Syndrome-associated Coronavirus (SARS-CoV)

Chow, Ken Yan Ching; Hon, Chung Chau; Hui, Raymond Kin Hi; Wong, Raymond Tsz Yeung; Yip, Chi Wai; Zeng, Fanya; Leung, Frederick Chi Ching

Together with the M protein, the spike protein is believed to be incorporated into the viral envelope before the mature virion is released (17 ) . Initial analysis of the 1255-a.a. peplomer protein of the virus reveals the possible existence of a signal peptide that would likely be cleaved between residues 13 and 14 (12 ) . The whole structure is predicted to contain a receptor-binding unit (S1) in the N-terminus (14 , 25 -27 ) and a transmembrane unit (S2) in the C-terminus (13 , 14 , 25 , 27 ) . Molecular modeling of the S1 and S2 subunits of the spike glycoprotein (26 , 28 ) suggested that the former unit is consisted of mainly anti-parallel β-sheets with dispersed α and β regions, in addition to the three domains identified in the S2 unit. The confidence level of the predicted molecular models was strengthened by the good correlation between predicted accessibility and hydropathy profiles and by the correct locations of the N/O-glycosylation sites and most of the disulfide bridges. Whether the experimentally determined N-glycosylated sites from purified spike protein treated by tryptic digest together with PNGase followed by time-of-flight (TOF) mass spectrometry (29 ) are correctly located in the proposed model remains to be clarified. In the aspect of biological activities, receptors for the binding of the SARS-CoV remain mysterious, as comparative genomics did not point out any significant similarity with the S1 domain of other human coronaviruses, implying that these viruses are using different receptors for cell entry (12 ) . Subsequently, angiotensinconverting enzyme 2 (ACE2) was demonstrated to be a functional receptor for the SARS-CoV in vitro. Synctia was observed in cell culture expressing ACE2 and the SARS-CoV S1 domain, which could be inhibited by anti-ACE2 antibody (30 ) . Fine mapping on the N-terminal unit of the spike protein indicates that the receptor-binding domain is probably located between the residues 303 and 537 (31 ) .