One possibility is that TAT does have a signal sequence but not in the classical sense. Proteins can have a non-classical nuclear localization signal and still be imported into the nucleus with the same nuclear import machinery (Nakada, R., Hirano, H. & Matsuura, Y). Another possibility is that TAT passes through the ER membrane by binding to another protein with an ER localization signal.

Nakada, R., Hirano, H. & Matsuura, Y. Structure of importin-α bound to a non-classical nuclear localization signal of the influenza A virus nucleoprotein. Sci Rep 5, 15055 (2015) doi:10.1038/srep15055

Thank you for helping

Dear Maram Balkhair ,

In order to see how and to which degree you want to answer your questions I strongly advise you to read:

Clark, E., Nava, B., & Caputi, M. (2017). Tat is a multifunctional viral protein that modulates cellular gene expression and functions. Oncotarget, 8(16), 27569.

https://pdfs.semanticscholar.org/1563/3dbc81eb0bda4727954b278b70ced11f65a5.pdf

This paper confirms that the mechanism of Tat secretion is poorly defined and depends on a leaderless secretory pathway that seems to be independent from the endoplasmic reticulum and the Golgi apparatus. The so-called Tat basic domain (the protein transduction domain (PTD)) is believed to facilitate the trafficking of Tat through the plasma membrane and functions as a cell penetrating peptide once conjugated to a protein cargo. Ultimately the nuclear localization signal (GRKKR) mediates translocation of Tat into the cell nucleus. The mechanism by which Tat penetrates the plasma membrane is believed to depend on its ability to recognize multiple binding sites at the cell surface.

See also the following interesting paper:

Nickel W and Rabouille C. Mechanisms of regulated unconventional protein secretion. Nat Rev Mol Cell Biol. 2009; 10:148-155

Hope this helps you a bit further.

Best regards.