P4. Abstract HIV-1 nuclear import is facilitated by the viral capsid protein (CA), interacting with several key cellular factors. However, the precise roles of each of these interactions in the trafficking of the viral RTC/PIC toward the nucleus and from the microtubule organizing center into the nucleus are poorly understood. We will utilize advanced imaging technology and specifically labeled viral and cellular proteins to track CA and RTC/PIC movements and interactions in real time, which will reveal hitherto unknown details of the early events of HIV- 1 biology that underlie viral trafficking, uncoating, nuclear import, and integration. We also will solve novel structures of viral-host protein complexes that are involved in the nuclear transport process, including CA in complex with NUP358, NUP153, and CPSF6 proteins, the latter also in complex with its β-karyopherin binding partner TNPO3. The completion of Project 4 will yield novel information on the virus-host interactions that underlie the key early events of the HIV-1 lifecycle at both the cell- and atomic levels.