[The cloning, expression and structural analysis of putative unknown protein Orf 9b in SARS-CoV]

Orf 9b was amplified by PCR from SARS-CoV genome and cloned into the Nco I and Bam HI sites of the pET32c expression vector, and then recombinant plasmid pET32c-Orf 9b was constructed. The recombinant fusion protein Orf 9b was expressed by IPTG induction and purifed. After being cleaved by rEK, Orf 9b protein with MW 11 kD was separated and collected. It was demonstrated by ELISA that the purified Orf 9b protein could react with sera of SARS rehabilitaion patients but not with sera from healthy donors. CD and Infrared spectroscopy were used to predict the secondary structure of the purified Orf 9b protein. The distribution percentages for the the secondary structures of alpha-helix, beta-sheet, and random coil in the Orf 9b protein estimated by CD were 12.5%, 40%, and 47.5%, respectively, while the same parameters estimated by Infrared spectroscopy were 13.7%, 47.5%, and 37.9%, respectively. The results obtained by the two methods were substantially identical and showed that the structure of the Orf 9b protein consisted mostly of beta-sheet, and comprised only few alpha-helix. The acquisition of purified protein and the structural information presented herein may provide foundation for further functional study.