These studies will continue a long-standing collaboration between laboratories at Cornell University and Purdue University which involve genetic, virological, and structural studies of parvovirus capsids, their cellular receptors, and the antibodies that neutralize them. Parvoviruses cause widespread and serious disease in humans and other animals, and are genetically and structurally simple. However, they display the behaviors that are seen for most animal viruses, including receptor binding, endosomal uptake and cell infection, cytoplasmic and nuclear trafficking, and efficient neutralization by antibodies. In most of the work proposed here we are studying canine parvovirus (CPV) and feline panleukopenia virus (FPV), which are >99.5% identical in DNA sequence, yet which differ in host range through mutations in the capsid protein which affect specific receptor binding. In our recent work we have defined capsid structural variation, capsid assembly processes, antibody-capsid interactions, and identified the transferrin receptor (TfR) as a major receptor for cell binding and infection. We will continue studies in 5 areas. (1) Mutagenesis of the feline TfR and the capsid to define the interactions between those molecules that control viral binding and allow infection; (2) Use cryo-electron microscopy and X-ray crystallography to define in detail the structures of the interacting sites on the feline and canine TfRs and the capsid; (3) Analyze structural variation of the capsid, and define changes induced by TfR binding leading to infection of cells, as well as those induced by antibody binding leading to neutralization. Antibody domains expressed on cells as fusions with the TfR will be used as alternative receptors to determine whether they can mediate virus infection of cells; (4) Examine the role of a pore in the capsid structure and determine whether it controls exposure of the N-termini of the VP1 or VP2 proteins, or the 3' or 5' ends of the viral DNA. The infectivity and release of sequences from the mutant capsids will be examined and compared to wild type viruses; (5) Determine the high resolution structure of the human parvovirus B19 capsid and correlate that structure with the known properties of the virus.