Yeast expression and characterization of SARS-CoV N protein

The severe acute respiratory syndrome human coronavirus (SARS-CoV) nucleocapsid protein (N protein) is its most antigenic structural protein. The N protein gene has been cloned into a yeast expression vector pPIC9, transformed into Pichia pastoris strain GS115 and induced for expression by methanol. SDS-PAGE and Western blot showed that the N protein was expressed at a level of 3 mg/ml of culture medium. Characterization by mass spectrometry, circular dichroism and fluorescence luminescence assays showed that the expressed N protein displayed a β-sheet secondary structure in solution and it is stable in the pH range between 5.0 and 8.0. The P. pastoris-expressed N protein is believed to more closely resemble native SARS N protein than the bacterially expressed N protein.