Steady-state nuclear localization of exportin-t involves RanGTP binding and two distinct nuclear pore complex...

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Exportin-t (Xpo-t) is a key receptor for the nuclear export of mature tRNAs in vertebrates.
• Xpo-t functions by forming a complex with RanGTP and cargo molecules, facilitating transport through nuclear pore complexes (NPCs).
• While Xpo-t shuttles bidirectionally, it predominantly resides in the nucleus at steady state, a distribution influenced by RanGTP binding.

Purpose of the Study:

• To elucidate the molecular mechanisms governing the steady-state distribution and nuclear pore complex interactions of exportin-t (Xpo-t).
• To identify specific domains within Xpo-t responsible for its interaction with nucleoporins and to characterize the dependence of these interactions on RanGTP.

Main Methods:

• In vitro binding assays were employed to analyze the interactions between Xpo-t domains and peripherally localized nucleoporins.
• The differential binding of Xpo-t's N- and C-termini to specific nucleoporins, including Nup153, RanBP2/Nup358, and CAN/Nup214, was investigated.
• The role of RanGTP in modulating these nucleoporin-Xpo-t interactions was assessed.

Main Results:

• Two distinct interaction domains within Xpo-t were identified, exhibiting differential binding affinities for nucleoporins.
• The N-terminal domain of Xpo-t binds to Nup153 and RanBP2/Nup358 in a RanGTP-dependent manner.
• The C-terminal domain of Xpo-t interacts with CAN/Nup214 independently of Ran binding.

Conclusions:

• The identified interactions of Xpo-t with nucleoporins, modulated by RanGTP, are crucial for its nuclear import and export cycle.
• These interactions likely concentrate tRNA export complexes and empty Xpo-t near NPCs, thereby enhancing the overall efficiency of the tRNA export pathway.
• Understanding these mechanisms provides insight into the regulation of nucleocytoplasmic transport of essential RNA molecules.