An Infectious Rous Sarcoma Virus Gag Mutant That Is Defective in Nuclear Cycling.

During retroviral replication, unspliced viral genomic RNA (gRNA) must escape the nucleus for translation into viral proteins and packaging into virions. "Complex" retroviruses, such as human immunodeficiency virus (HIV), use -acting elements on the unspliced gRNA in conjunction with -acting viral proteins to facilitate this escape. "Simple" retroviruses, such as Mason-Pfizer monkey virus (MPMV) and murine leukemia virus (MLV), exclusively use -acting elements on the gRNA in conjunction with host nuclear export proteins for nuclear escape. Uniquely, the simple retrovirus Rous sarcoma virus (RSV) has a Gag structural protein that cycles through the nucleus prior to plasma membrane binding. This trafficking has been implicated in facilitating gRNA nuclear export and is thought to be a required mechanism. Previously described mutants that abolish nuclear cycling displayed enhanced plasma membrane binding, enhanced virion release, and a significant loss in genome incorporation resulting in loss of infectivity. Here, we describe a nuclear cycling-deficient RSV Gag mutant that has similar plasma membrane binding and genome incorporation to wild-type (WT) virus and surprisingly is replication competent, albeit with a slower rate of spread than observed in WT virus. This mutant suggests that RSV Gag nuclear cycling is not strictly required for RSV replication. While mechanisms for retroviral Gag assembly at the plasma membrane are beginning to be characterized, characterization of intermediate trafficking locales remain elusive. This is in part due to the difficulty of tracking individual proteins from translation to plasma membrane binding. Rous sarcoma virus (RSV) Gag nuclear cycling is a unique phenotype that may provide comparative insight to viral trafficking evolution and may present a model intermediate to - and -acting mechanisms for gRNA export.