The Carboxyl-Terminus Directs TAF_I48 to the Nucleus and Nucleolus and Associates with Multiple Nuclear Import Receptors

Abstract

The protein complex Selectivity Factor 1, composed of TBP, TAF_I48, TAF_I63 and TAF_I110, is required for rRNA transcription by RNA polymerase I in the nucleolus. The steps involved in targeting Selectivity Factor 1 will be dependent on the transport pathways that are used and the localization signals that direct this trafficking. In order to investigate these issues, we characterized human TAF_I48, a subunit of Selectivity Factor 1. By domain analysis of TAF_I48, the carboxyl-terminal 51 residues were found to be required for the localization of TAF_I48, as well as sufficient to direct Green Fluorescent Protein to the nucleus and nucleolus. The carboxyl-terminus of TAF_I48 also has the ability to associate with multiple members of the β-karyopherin family of nuclear import receptors, including importin β (karyopherin β1), transportin (karyopherin β2) and RanBP5 (karyopherin β3), in a Ran-dependent manner. This property of interacting with multiple β-karyopherins has been previously reported for the nuclear localization signals of some ribosomal proteins that are likewise directed to the nucleolus. This study identifies the first nuclear import sequence identified within the TBP-Associated Factor subunits of Selectivity Factor 1.