Laguna Design / Science Photo Library. SARS virus capsid protein, molecular model. This protein is responsible for binding the capsid (outer coat) of the SARS (severe acute respiratory syndrome) coronavirus to the virus's RNA (ribonucleic acid), which holds its genetic code. This property helps the viruses assemble as they are manufactured within a host cell. The virus caused a global outbreak of a pneumonia-like disease in 2003. In this model, solid colours represent the secondary structure of the protein (coils are alpha helices; arrows are beta sheets) and the faded tubular structure behind represents the individual atoms and bonds.