Nuclear export of cellular mRNAs is highly selective, wherein only fully processed RNAs are exported. Retroviral replication requires export of unspliced viral RNAs, which serve as templates for structural proteins and as genomic RNA for progeny virus. The export of unspliced viral RNA depends on the interaction of cis-acting RNA elements with cellular factors. In simian type D retroviruses, the cis-acting element is the constitutive transport element (CTE). CTE-dependent nuclear transport is mediated by the cellular TAP protein. TAP (Tip associating protein) is the vertebrate homolog of the yeast nuclear export protein Mex67p. TAP is a nuclear protein that shuttles between the nucleus and cytoplasm. It contains an N-terminal nuclear localization and export region (NLS-NES), a central RNA binding domain (RBD), and a C-terminal portion that is required for nuclear localization and nuclear rim association. The first 372 aa of TAP are necessary and sufficient for binding and nuclear export of CTE-containing RNA. Thus, TAP contains a novel RNA-binding motif that recognizes CTE-containing RNA and interacts with other components of the nuclear transport machinery.
