Identification of Novel Import and Export Signals of Human TAP, the Protein That Binds to the Constitutive Transport Element of the Type D Retrovirus mRNAs

Abstract

The nuclear export of the unspliced type D retrovirus mRNA depends on the cis-acting constitutive transport RNA element (CTE) that has been shown to interact with the human TAP (hTAP) protein promoting the export of the CTE-containing mRNAs. We report here that hTAP is a 619-amino-acid protein extending the previously identified protein by another 60 residues at the N terminus and that hTAP shares high homology with the predicted rat and mouse TAP proteins. We found that hTAP is a nuclear protein that accumulates in the nuclear rim and the nucleoplasm. We further demonstrated that hTAP is able to shuttle between the nucleus and the cytoplasm. Identification of the signals responsible for nuclear import (NLS) and export (NES) revealed that they are distinct but partially overlapping. NLS and NES of hTAP are active transferable signals that do not share similarities with known elements. The C-terminal portion contributes further to hTAP’s nuclear retention and contains a signal(s) for nuclear rim association. Taken together, our data show that hTAP is a dynamic protein capable of bidirectional trafficking across the nuclear envelope. These data further support hTAP’s role as an export factor of the CTE-containing mRNAs.

ACKNOWLEDGMENTS

We thank E. Izaurralde for sharing unpublished data, reagents, and discussions. We are grateful to E. Afonina, K. Horie, A. Gragerov, G. N. Pavlakis, and T. Hope for reagents and discussions, to G. Gragerova for technical assistance, and to C. Rhoderick for secretarial assistance.

This research sponsored by the National Cancer Institute, Department of Health and Human Services, under a contract with ABL.

J.B., W.T., and A.S.Z. contributed equally to this work.

ADDENDUM IN PROOF

In agreement with our data, a recent paper by Katahira et al. (EMBO J. 18:2593–2609, 1999) also describes that hTAP associates with the nuclear rim, where they find TAP to interact with nucleoporins.