The human polyomaviruses, JCV and BKV, establish lifelong persistent infection in kidney but are generally not associated with disease in healthy individuals. In immunosuppressed patients reactivation and spread of JCV to the central nervous system causes a fatal demyelinating disease known as progressive multifocal leukoencephalopathy (PML). Reactivation of BKV in immunosuppressed renal transplant patients causes polyomavirus associated nephropathy (PVN) that leads to graft dysfunction and loss. Viral DMA and viral gene products from both JCV and BKV have been detected in human cancers but a causal link has not been established. Our long-term goals are to define the role of virus receptors in cellular invasion, spread, and pathogenesis. During the last funding cycle we discovered that the JCV receptor is a complex consisting of alpha(2,6) linked sialic acid and the 5HT2a receptor. Both components are critical for infection as cells missing either component are not susceptible to infection by JCV. Our efforts are now focused on identifying the critical determinants that allow JCV to productively interact with this receptor complex. As the 5HT2a receptor is a G protein coupled receptor and JCV binding to cells induces a signal that is essential for infection we will determine whether JCV signals directly through this receptor. During the last funding cycle we discovered that the receptor for BKV is an N-linked glycoprotein containing alpha(2,3)-linked sialic acid, that signaling is important for BKV infection, and that BKV does not share receptor specificity with JCV. A detailed molecular genetics approach is proposed to identify the critical residues in the BKV capsid protein VP1 that interacts with sialic acid containing receptors. We will also compare JCV and BKV induced signaling and entry pathways in the cell. Our specific aims are to 1. define the interactions between human polyomavirus capsid proteins and host cell receptors, 2. define the molecular pathways that link virus- induced signaling at the plasma membrane to transcriptional responses in the nucleus, and 3. map the infectious entry pathway leading to the nucleus and identify the viral and cellular proteins involved at each step. Data resulting from these studies will yield novel insights into the pathogenesis of human polyomavirus induced disease and may lead to novel therapies to prevent or treat these diseases.