We have employed NMR to investigate the structure of SARS coronavirus nucleocapsid protein dimer. We found that the secondary structure of the dimerization domain consists of five ฮฑ helices and a ฮฒ-hairpin. The dimer interface consists of a continuous four-stranded ฮฒ-sheet superposed by two long ฮฑ helices, reminiscent of that found in the nucleocapsid protein of porcine respiratory and reproductive syndrome virus. Extensive hydrogen bond formation between the two hairpins and hydrophobic interactions between the ฮฒ-sheet and the ฮฑ helices render the interface highly stable. Sequence alignment suggests that other coronavirus may share the same structural topology. ยฉ 2005 Published by Elsevier B. V. on behalf of the Federation of European Biochemical Societies.