Coronaviruses are responsible for about 30% of human upper respiratory infections each year. In 2002, a new type of coronavirus (SARS-CoV) caused a sever outbreak in 29 countries or regions worldwide. Understanding the virus life cycle and how it spreads is essential for rational drug design. The objective of this application is to understand how the coronavirus genomic RNA is recognized and packaged into the progeny virion. The central hypothesis is that the nucleocapsid protein (N) functions as a scaffold, binding RNA non-specifically, while the membrane protein (M) selectively packages viral genomic RNA. In order to gain insight into the viral packaging mechanism, the proposed research focused on a well characterized coronavirus, mouse hepatitis virus (MHV). Since the sequence of the SARS-CoV genome suggests that it is closely related to MHV, studies into MHV will also advance our understanding of the biology of SARS-CoV. Specifically we will: 1. Characterize the self-association property of the nucleocapsid protein; 2. Characterize the interactions between the viral RNA, the membrane protein, and the nucleocapsid protein; and 3. Determine the structure of the protein/RNA complex by X-ray crystallography or electron microscopy. At the completion of the proposed studies, we expect that we will have gained new knowledge of how coronavirus assembles inside an infected cell. Such knowledge will help us to identify major targets for antiviral drug development. The severe acute respiratory syndrome (SARS) caused by coronavirus is an emerging disease of serious health concern. This application aims at establishing a general model for coronavirus assembly. Knowledge gained in this study will be useful for the development of antiviral drugs. ? ? ?