Severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) caused a severe outbreak in several regions of the world in 2003. The virus is a novel coronavirus isolated from patients exhibiting atypical pneumonia and may have originated from wild animals such as civet cats in southern China. The genome of SARS-CoV is a positive-sense, single-stranded RNA whose sequence is distantly related to all known coronaviruses that infect humans and animals. Like other known coronaviruses, SARS-CoV is an enveloped virus containing three outer structural proteins, namely the membrane (M), envelope (E), and spike (S) proteins. The nucleocapsid (N) protein together with the viral RNA genome form a helical core located within the viral envelope.
The severe acute respiratory syndrome coronavirus nucleocapsid protein (SARS-CoV NP), a phosphoprotein of 48 kDa, is the most abundant protein in the virus-infected cells. Its primary function is to package the ∼30 kb single stranded, 5′-capped positive strand viral genome RNA molecule into a ribonucleoprotein (RNP) complex called the capsid. Ribonucleocapsid packaging is a fundamental part of viral self-assembly and the RNP complex constitutes the essential template for replication by the RNA-dependent RNA polymerase complex. In addition, the N-protein of the SARS-CoV has been shown to modulate the host cellular machinery and may serve regulatory roles during its viral life cycle.
As the most abundantly expressed structural protein during infection, SARS-CoV NP is highly detectable in SARS patients. Therefore, this protein may serve as one of the immunodominant antigens in the early diagnosis of infection. Furthermore, researchers have suggested that antibody against the N protein could modulate cytokine responses such as IL-11; non-neutralizing antibodies against N protein were found to protect mice against lethal infection.