gms | German Medical Science

We have constructed and expressed recombinant fusion proteins containing human IgG Fc fragment and truncated spike (S) glycoproteins of SARS-CoV. With these recombinant fusion proteins and overlapping peptides covering the entire S protein, we had identified three distinct receptor-binding domains of protein S. Using VERO E6 cell as target; we identified and mapped two separated receptor-binding domains of S protein. The low affinity binding domain (>10 μM) located within the N-terminal 333 residues and its receptor was identified to be ACE2 protein reported by W. Li et al. [Ref. 1], the high affinity-binding domain (~1 nM) was mapped to residues 334 to 666 and also bound strongly to ACE2 protein. This strong binding could be partially inhibited by peptides containing residues 576 to 603. In addition, rabbit antisera raised against peptide fragment corresponding to residues 433 to 467, could completely block S protein binding to VERO E6 cells. When NIH/3T3, HeLa, BHK-21 and COS-7 cells as targets, we found three separated receptor-binding domains of S protein: the low affinity was mapped again to the N-terminal 333 residues, the mild binding site (1 μM) was mapped to residues 334 to 666, but the high affinity (10 nM) is located within residues 667 to 999. All these results suggest that SARS CoV could potentially have different tropism and use different functional receptor(s) for viral entry. This multiple potential receptors have significant implications that new high infectious SARS-like mutants can emerge through acquiring the mutation of interest through recombination or deletions of spike gene. Recent reports had demonstrated that crossing the host cell species barrier by substitution of the coronavirus spike glycoprotein ectodomain.

Based on all current information, we propose a model for the SARS CoV spike protein and its potential receptors interaction. The binding domains of ACE2 like clamps and hold on SARS spike protein at two regions (residues 1- 333 and 524 to 618). The interactions are further stabilized by the electrostatic forces from charge residues and hydrophobic patch created by the aromatic residues of the loops or ridge structure of spike protein at region 433 to 467, inserting into or laying onto the catalytic site (cavity structure) of ACE2. Potential receptor X found in other cell lines could further involving the region containing residues 776-825 where was found to comprise of a major human B-cell epitope [Ref. 2]. Mutational analysis of S protein will elucidate the molecular determinants of cell-specific and species-specific recognition of these receptors by S protein of SARS CoV.

This research was supported by National Science Council of Republic of China Grants # SVAC12-06 (P. Chong), SIMM07-00 and SIMM07-01 (E. Hsieh).