Using viral species specificity to define a critical protein/RNA interaction surface.

The Tap protein mediates the sequence-specific nuclear export of mRNAs bearing the retroviral constitutive transport element (CTE) and also plays a critical role in the sequence nonspecific export of cellular mRNAs. Previously, we have demonstrated that CTE function displays species specificity, that is, the CTE functions in human but not quail cells. Here, we demonstrate that quail Tap fails to support CTE function because it cannot bind the CTE. However, changing a single residue in quail Tap, glutamine 246, to arginine, the residue found in human Tap, rescues both CTE function and CTE binding. This residue, which is located on the exterior of a recently reported molecular structure of Tap, defines a surface on Tap that is critical for CTE binding. These data emphasize the potential importance of cross-species genetic complementation in the identification and characterization of cellular factors that are critical for different aspects of viral replication.