PHAX, a mediator of U snRNA nuclear export whose activity is regulated by phosphorylation

In metazoa, assembly of spliceosomal U snRNPs requires nuclear export of U snRNA precursors. Export depends upon the RNA cap structure, nuclear cap-bi nding complex (CBC), the export receptor CRM1/Xpo1, and RanGTP. These compo nents are however insufficient to support U snRNA export. We identify PHAX (phosphorylated adaptor for RNA export) as the additional factor required f or U snRNA export complex assembly in vitro. In vivo, PHAX is required for U snRNA export but not for CRM1-mediated export in general. PHAX is phospho rylated in the nucleus and then exported with RNA to the cytoplasm, where i t is dephosphorylated. PHAX phosphorylation is essential for export complex assembly while its dephosphorylation causes export complex disassembly. Th e compartmentalized PHAX phosphorylation cycle can contribute to the direct ionality of export.