Spike (S) proteins of coronaviruses are large transmembrane heavily N-glycosylated proteins that mediate association with a cell surface receptor (Li et al. 2006). The SARS-CoV S protein possesses 23 N-linked glycosylation sites distributed in three clusters. The glycosylation of 13 of these sites has been confirmed (Krokhin et al. 2003; Ying et al. 2004; Chakraborti et al. 2005). Receptor binding domains (RBDs) have been identified in the S1 domain of a number of coronaviruses, and a fragment of the SARS-CoV S1 domain, from residues 318–510, binds human angiotensin-converting enzyme 2 (ACE2) with high affinity (Xiao et al. 2003; Babcock et al. 2004; Wong et al. 2004). It has additionally been demonstrated that ACE2 constitutes an obligate cellular receptor although other receptors may participate in the infection process (Li et al. 2003; Chen and Subbarao 2007). The structure of SARS-CoV RBD complexed with ACE2 revealed that an extended loop of the RBD, comprising residues 424–494, is in direct contact with ACE2 (Li et al. 2005). This receptor binding motif (RBM) is not glycosylated, but it is surrounded by two clusters of glycosylation sites (Han et al. 2007).