EXCEED THE SPACE PROVIDED. Our long term goal is to understand the molecular mechanisms by which viruses occasionally jump from their normal host species to a new species, a process that may lead to emerging viral diseases in the new host species. We will focus on the interactions of the novel type 1 fusion glycoprotein (S) of murine coronavirus MHV with host cell receptors in the CEA family of glycoproteins. Persistent infection of murine cell lines with MHV leads to markedly reduced expression of the CEACAMla receptor isoforms. In the persistently infected cultures, mutant viruses with mutations in their spike glycoproteins and some other genes rapidly replace the wild type virus. The mutations in the spike genes are associated with acquisition of the ability of the virus to replicate in cell lines from cats, pigs, rats, monkeys and humans. We will identify amino ace residues of the viral spike glycoprotein that bind to the murine CEACAMla proteins, and amino acids of the receptor that bind to the spike. We will study the effects of mutations in S of MHV that make it resemble related coronaviruses that replicate in human, rat or bovine cells but not murine cells. We will analyze how mutations in the receptor that make it resemble CEACAM1 of other species affect the specificity of virus-receptor interactions. We will continue our work on determining the crystal structures of CEACAM1 proteins to learn how the functional CEACAMla receptor differs from CEACAMlb proteins from MHV-resistant mice and from human CEACAM1 and to engineer spike and receptor proteins that will form co-crystals. We will analyze the conformational changes in the viral spike protein that are induced by soluble receptor or pH8 at 37C. To develop strains of inbred mice that are resistant to MHV infection, we will manipulate the ceacaml gene in mice to reduce or eliminate expression to CEACAM1 proteins, and to substitute chimeras between CEACAM la and CEACAM lb for the normal gene in mice. The animals will be tested for MHV-susceptibility and immune responses. These studies will provide important information about the mechanism of changing receptor specificity and virus entry in a fascinating model virus that causes hepatitis, enteritis, immune dysfunction and demyelinating neurological disease in its natural host. PERFORMANCE SITE ========================================Section End===========================================