Intracellular localization of the SARS coronavirus protein 9b: evidence of active export from the nucleus.

Open reading frame 9b (ORF 9b) encodes a 98 amino acid group-specific protein of severe acute respiratory syndrome (SARS) coronavirus (CoV). It has no homology with known proteins and its function in SARS CoV replication has not been determined. The N-terminal part of the 9b protein was used to raise polyclonal antibodies in rabbits, and these antibodies could detect 9b protein in infected cells. We analyzed the sub-cellular localization of recombinant 9b protein using fluorescence microscopy of live transfected cells and indirect immunofluorescence of transfected fixed cells. Our findings indicate that the 9b protein is exported outside of a cell nucleus and localizes to the endoplasmic reticulum. Our data also suggest that the 46-LRLGSQLSL-54 amino acid sequence of 9b functions as a nuclear export signal (NES).