Nuclear targeting determinants of the far upstream element binding protein, a c-myc transcription factor

FUSE binding protein (FBP) binds in vivo and in vitro with:the single-stranded far upstream element (FUSE) upstream of the c-myc gene, In addition to its transcriptional role, FBP and its closely related siblings FBP2 (KSRP) and FBP3 have been reported to bind-RNA and participate in various steps of RNA processing, transport or catabolism. To perform these diverse-functions, FBP must traffic to different nuclear-sites, To identify determinants of nuclear localization, full-length FBP or fragments thereof were fused to green fluorescent protein, Fluorescent-FBP-localized in the nucleus in three patterns, diffuse, dots and spots, Each pattern was conferred by a distinct nuclear localization signal (NLS): a classical bipartite: NLS in the N-terminal and two non-canonical signals, an alpha -helix in the third KH-motif of the nucleic acid binding domain and a tyrosine-rich motif in the C-terminal transcription activation domain, Upon treatment with the transcription inhibitor actinomycin D, FBP completely re-localized into dots, but did not exit from the nucleus, This is in contrast to many general RNA-binding proteins, which shuttle from the nucleus upon treatment with actinomycin D, Furthermore, FBP co-localized with transcription sites and with the general transcription factor TFIIH, but not:with the splicing factor SC-35, Taken together, these data reveal complex intranuclear trafficking of FBP and support a transcriptional role for this protein.