The evolutionarily conserved region of the U snRNA export mediator PHAX isa novel RNA-binding domain that is essential for U snRNA export

In metazoa, a subset of spliceosomal U snRNAs are exported from the nucleus after transcription. This export occurs in a large complex containing a U snRNA, the nuclear cap binding complex (CBC), the leucine-rich nuclear expo rt signal receptor CRM1/Xpo1, RanGTP, and the recently identified phosphopr otein PHAX ((ph) over bar osphorylated (a) over bar daptor for RNA e (x) ov er bar port), Previous results indicated that PHAX made direct contact with RNA, CBC,and Xpo1 in the U snRNA export complex, We have now performed a s ystematic characterization of the functional domains of PHAX, The most evol utionarily conserved region of PHAX is shown to be a novel RNA-binding doma in that is essential for U snRNA export. In addition, PHAX contains two maj or nuclear localization signals (NLSs) that are required for its recycling to the nucleus after export, The interaction domain of PHAX with CBC is at least partly distinct from the RNA-binding domain and the NLSs. Thus, the d ifferent interaction domains of PHAX allow it to act as a scaffold for the assembly of U snRNA export complexes.