Phosphorylated Adapter RNA Export Protein (PHAX)

Export depends upon the RNA cap structure, nuclear cap-binding complex , the export receptor CRM1 (XPO1), and RanGTP. These components are, however, insufficient to support U snRNA export. In vivo, Phax is required for U snRNA export but not for Crm1-mediated export in general. Phax is phosphorylated in the nucleus and then exported with RNA to the cytoplasm, where it is dephosphorylated. Phax phosphorylation is essential for export complex assembly, while its dephosphorylation causes export complex disassembly. The compartmentalized Phax phosphorylation cycle can contribute to the directionality of export. Several human ESTs share more than 90% homology with the mouse Phax gene, indicating that a homologous PHAX gene exists in the human genome.