Infection cycles of the human, animal, and plant viruses include processes of virion assembly and dissemination within the infected organism. These processes require functions of the specialized viral capsid and transport proteins, as well as cellular proteins and organelles recruited by virus. The functional coordination among viral and host components during virion formation and spread are understood relatively poorly. The beet yellows virus (BYV) model was developed during this project to reveal interactions between the viral and cell proteins using innovative biochemical, genetic and cell biological approaches. The BYV homolog of Hsp70 molecular chaperones (Hsp70h) was found to integrate principal functions of virion assembly and transport with modifications of the cell architecture. The BYV model provides unique advantages for investigation of these processes, including ability to visualize virus-host interactions within the live infected cells. Two Specific Aims are in the focus of this proposal. Aim 1 is designed to determine the molecular architecture of the BYV virions. These virions possess the tails, which serve as specialized movement devices empowered by Hsp70h. The topology of the tail proteins will be resolved using electron and atomic force microscopy. The Hsp70h-guided mechanism of the tail assembly will be dissected using available in planta model. Aim 2 will address the interactions between Hsp70h and the host cell. The hypothesis stating that Hsp70h interacts with host proteins to remodel cellular environment and facilitate virus trafficking in association with the cytoskeletal motility systems will be tested. Using BYV variants that express organelle-targeted fluorescent reporters, real-time changes in cell architecture during virus infection will be examined. Host proteins that interact with Hsp70h will be identified using yeast two-hybrid screens and mass spectrometry analyses. Functional significance of the observed interactions will be assessed using genetic strategies targeting both Hsp70h and host proteins.