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 export
signal receptor CRM1/Xpo1, RanGTP, and the recently identified
phosphoprotein PHAX (phosphorylated adaptor
for RNA export). Previous results indicated that
PHAX made direct contact with RNA, CBC, and Xpo1 in the
U snRNA export complex. We have now performed a systematic
characterization of the functional domains of PHAX. The
most evolutionarily conserved region of PHAX is shown to
be a novel RNA-binding domain that is essential for U snRNA
export. In addition, PHAX contains two major 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 different interaction domains of
PHAX allow it to act as a scaffold for the assembly of
U snRNA export complexes.