The spike (S) protein of SARS-CoV is a major virion structural protein. It plays an important role in cell tropism, specific binding with receptor, and cell-membrane fusion [1] [2] [3] [4] . The spike protein is of good antigenicity and could induce neutralization antibodies. Among the structural proteins, the spike protein is the only significant neutralization antigen and protective antigen. Hence, it is a suitable candidate for genetic engineering of a subunit vaccine [5] [6] [7] [8] . The development of SARS-CoV spike protein specific monoclonal antibodies (MAbs), especially that of neutralizing MAbs, are important in the prevention and diagnoses of SARS. Some research groups have reported MAbs of SARS-CoV [9] [10] [11] [12] . In a structural and functional study of spike protein, identification of antigenic epitopes is important. Especially, identification of neutralizing epitopes may be beneficial for designing vaccine. To date, many linear antigenic epitopes are documented. Among these linear epitopes, some are neutralizing epitopes [13] [14] [15] [16] . But on the spike protein, especially on the receptor-binding domain of the spike protein, most protective neutralizing epitopes are conformational epitopes [17] . Usually it is difficult to determine the specific sequence of discontinuous conformational epitopes. However, a phage-displayed peptide library could be used to select the mimotope that could mimic the discontinuous or conformational epitopes. Here the phage-displayed peptide library was used to select the mimotope of a SARS-CoV spike protein specific neutralizing MAb 2C5. The results may provide information for further structural and functional research and for designing SARS vaccines. Phages are titered according to the kit manual as follows: First, inoculate 5-10 mL of LB with a single clone of ER2738 and incubate with shaking until a mid-log phase (OD 600~0 .5). When cells grow, melt agarose top in microwave and dispense 3 mL into sterile culture tubes, one per expected phage dilution. Equilibrate tubes at 50 °C until they are ready for use. Dilute phage in 10-fold serial dilution with LB. Once culture has reached the mid-log phase, dispense 200 µL of culture into microfuge tubes, one for each phage dilution. Add 10 µL of each dilution to each tube, vortex rapidly, and incubate at room temperature for 5 min. Transfer the infected cells, one at a time, to a culture tube containing 50 °C agarose top, vortex rapidly, and immediately pour onto a 37 °C pre-warmed LB/Tet/IPTG/X-Gal plate. Allow the plate to cool for 5 min, invert, and incubate overnight at 37 °C. Examine the plates and count plaques on the plate that contains about 100 plaques. Convert phage titers to plaque forming units (pfu) per 10 µL by multiplying each number by the dilution factor.
Panning the peptide library against MAb 2C5 is carried out according to the kit manual. The main procedure is as follows: Microtiter plates are coated with MAb 2C5 in 0.1 mol/L NaHCO 3 (pH 8.6) at 4 °C overnight and are blocked at 4 °C for 2 h with blocking buffer. Wash plates five times with TBST. After washing, immediately follow panning and avoid drying out of the plates. Dilute 2×10 11 phage with 100 µL TBST, pipette onto coated plates, and shake gently for 60 min at room temperature. Discard nonbinding phage by pouring off and tapping the plate face down onto a clean paper towel. Wash plates 10 times with TBST. Use a clean part of the paper towel each time to prevent cross-contamination. Elute bound phage with 100 µL elution buffer. Shake gently for 10 min at room temperature. The solution in the pipette was eluted into a microcentrifuge tube and neutralized with 15 µL 1mol/L Tris·HCl (pH 9.1). Titer 1 µL of the eluate as described above. Add the remaining eluate to 20 mL of ER2738 culture and incubate at 37 °C with vigorous shaking for 4.5 h. The amplified phage was used for the second round of panning. After three rounds of panning, titer the eluate and randomly take 20 plaques from the plates in which the total plaques are less than 100. These phage clones were amplified and designated 2C5P1, 2C5P2 to 2C5P20.
The randomly taken 20 phage clones were sequentially analyzed after amplification. The sequencing templates were rapidly purified as follows: After plaque amplification and first centrifugation, transfer 500 µL of the phage-containing supernatant to a fresh microfuge tube. Add 200 µL PEG/NaCl. Invert to mix, and allow it to stand at room temperature for 10 min. Centrifuge under 12 000 g for 10 min and discard supernatant. Suspend pellet thoroughly in 100 µL iodide buffer and add 250 µL ethanol. Incubate for 10 min at room temperature. Spin under 12 000 g for 10 min and discard supernatant. Wash pellet in 70 % ethanol, and dry for a short period of time in vacuum. Suspend pellet in 30 µL distilled water and use for sequencing templates. The sequencing primer is M13-96g11, and the sequence is 5′-GCCCTC ATAGTTAGCGTAGCT-3′. All phage sequencing works are completed by Invitrogen Co. Ltd (Shanghai, China).
Microtiter plates were coated with MAb 2C5 in 10 µg/mL, 100 µL per well at 4 °C overnight. After being washed thrice with TBST (TBS contains 0.5 %Tween-20), the plates were blocked with 5 mg/mL BSA in 37 °C for 1 h and then washed thrice. Dilute the purified phage into 10 12 pfu/mL with TBS. Add 100 µL into one well, and for each sample, repeat the procedure for the three wells. Incubate at room temperature for 2 h and then wash five times. Add 100 µL 1:5 000 diluted HRP-conjugated mouse anti-phage M13 antibody to each well and incubate at 37 °C for 1 h, and wash five times. Color developed with the addition of o-phenylenediamine dihydrochloride and hydrogen peroxide. The reaction was stopped by the addition of 2 mol/L H 2 SO 4 and the absorbance was measured at 490 nm by microplate autoreader (Bio-Rad).
In competitive ELISA, the microtiter plates were coated with recombinant baculovirus-expressed spike protein. Before being incubated with coated microtiter plates, the primary antibody MAb 2C5 was first incubated with serial diluted phage 2C5EP14. The wild-type phage VcsM13 was used as the control. HRP-conjugated goat antimouse IgG was used as the secondary antibody.
To improve the efficiency and specificity of selection, the concentration of Tween-20 in washing buffer was increased during each round of panning. After three rounds of panning, the phage-displayed library resulted in an enrichment of phages that were bound to MAb 2C5. The output to input ratios of the three rounds of panning are shown in Table 1 .
After the third round of panning, 20 phage clones were randomly taken. To analyze the binding ability of these phages to the MAb 2C5, phage ELISA was conducted. The results revealed that among the OD values of 20 phage clones, there are ten over 0.2, and eight over 0.25 (Fig. 1) . The results demonstrated that after three rounds of panning, phagedisplayed peptides that can bind MAb 2C5 were enriched. 
The sequencing results show that among the 20 randomly selected phage clones, eight clones displayed the same sequence peptide TPEQQFT. The phage ELISA OD values of these eight phage clones are over 0.25 ( Table 2 ). The sequence analysis results were correct and coincided with those of phage ELISA. So the peptide TPEQQFT may mimic the epitope of MAb 2C5, and the peptide TPEQQFT is the mimotope of MAb 2C5. For further analysis of the epitope of MAb 2C5 on the spike protein, the peptide sequence was aligned with the spike protein sequence. And the alignment results showed that the peptide TPEQQFT may distribute in amino acid residues 539-559 (Fig. 2) on the spike protein of SARS-CoV. To confirm whether this region was the epitope of MAb 2C5 needs further experimental proof.
Phage ELISA and sequence analysis results demonstrated that peptide TPEQQFT may be a mimotope of MAb 2C5. The results of phage competitive ELISA also verified the binding ability of phage clone 2C5P14 to MAb 2C5. The phage clone 2C5P14 displayed peptide TPEQQFT. When the amount of phage reached 10 11 pfu per well, the binding of MAb with spike protein was nearly completely inhibited (Fig. 3 ). 
In 1985, Smith [18] first inserted foreign DNA fragments into the filamentous phage gene. The fusion protein was incorporated into the virion, which retains infectivity and displays the foreign amino acids in an immunologically accessible form. The foreign amino acids displayed on the phage surface could be purified by affinity chromatography to antibody directed against the gene product [19] . Peptides displayed on the surface of phage could retain its native structure and bioactivity. The fusion phage could be affinity-purified and selected. Therefore, it provided a powerful method for selected target proteins and peptides. Phage-display technique could be used to assess the interaction between proteins and peptides. And this technique now considerably benefits the research in designing new vaccine, and developing diagnostic reagents, antigenic epitope mapping, cell-surface engineering, and other affinity molecular purification [20] [21] [22] .
Phage-display technique has been used to create a physical linkage between a vast library of random peptide sequences to the DNA encoding each sequence, allowing rapid identification of peptide ligands for a variety of target molecules by an in vitro selection process called panning. In this study, the MAb 2C5 was used as the selection target. The panning was carried out by incubating the Ph.D.-C7C TM Phage-Displayed Peptide Library with the microtiter plate coated with the target. After three rounds of panning, individual clones are characterized by DNA sequencing and phage ELISA. Among 20 clones, nearly 10 displayed peptide TPEQQFT. By aligning with the spike protein of SARS-CoV, the peptide TPEQQFT may distribute in amino-acid residues 539 to 559. But the fusion protein containing spike-protein fragment of amino-acid residues 539 to 559 could be recognized by MAb 2C5. However, this fragment contains a linear epitope because this fusion protein could be recognized by SARS-CoV-immunized animal sera [14] . The phagedisplayed peptide TRPQQFT could inhibit the binding between MAb 2C5 and the spike protein. So this peptide is the mimotope of MAb 2C5.
In Western blot analysis, MAb 2C5 could not recognize the reduced cell cultured SARS-CoV spike protein, recombinant spike protein, and recombinant spike protein fragments. So the epitope of MAb was not a linear epitope, and its epitope was dependent on its conformation. Recently, reports about the characteristics of MAb 2C5 also confirmed that the epitope of MAb 2C5 was a conformational epitope [23, 24] . Monoclonal antibody 2C5 could neutralize SARS-CoV. It could inhibit the binding of spike protein receptor-binding domain with receptor ACE2. In this study, a peptide which could mimic the epitope of MAb 2C5 was identified. This result could provide useful information for further analyzing the function of spike protein and for the development of effective SARS vaccine.