Although current treatments have improved the lives of patients wit cystic fibrosis (CF), none are directed at the underlying defect and CF remains a lethal disease. Gene transfer is attractive because it could correct all of the physiologic abnormalities due to the loss of CFTR. Adeno-associated viruses have been studied for gene therapy. However, the efficiency of AAV2 gene transfer to human airway epithelia is low. We found that recombinant AAV5 binds to the apical surface and mediates gene transfer 50-fold more efficiently than AAV2. Our work indicates that gene transfer with AAV5 to the airway epithelia is receptor-mediated and that sialic acids in a 2,3 N-linked configuration are required for infection. Recent work by Chiorini's laboratory has identified the platelet derived growth factor receptors (PDGF-R), as receptors for AAV5. Thus, both sialic acids in a 2,3 N- linked configuration and PDGF-Rs are required for AAV5 transduction of cells. In this Grant application we propose to study the molecular interactions between AAV5 capsid and its receptors. We propose 3 specific aims: Are PDGF-Rs the receptors responsible for AAV5 binding and internalization in human airway epithelia? We hypothesize that N-glycosylation of PDGF-R is necessary for AAV5 binding and internalization. Alternatively, we hypothesize that 2,3 N -linked sialic acids in another membrane protein are required for AAV5 binding with the PDGF-Rs acting as a co-receptor required for internalization or other subsequent steps of infection. Does binding of AAV5 result in PDGF-Rs dimer formation and signaling? We hypothesize that AAV5 binding to PDGF-Rs results in receptor dimerization and phosphorylation and that phosphorylation of PDGF-Rs plays a role in AAV5 infection of cells. What regions of the capsid of AAV5 are required for binding to sialic acid and PDGF? Based on structure-function analysis of other members of the parvovirus family, and the 15 Angstroms cryo-EM structure of AAV5, we have identified a region in the capsid protein of AAV5 that is accessible to the surface and may play a role in sialic acid-mediated cell binding. We hypothesize that this region of AAV5 capsid is responsible for AAV5 binding to cells and gene transfer. Moreover we will investigate the region of AAV5 that binds to PDGFR.