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-binding complex (CBC), the export receptor CRM1/Xpo1, and RanGTP. These components are however insufficient to support U snRNA export. We identify PHAX (phosphorylated adaptor for RNA export) as the additional factor required for 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 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.
Ohno
M
M
European Molecular Biology Laboratory, Heidelberg, Germany.
Segref
A
A
Bachi
A
A
Wilm
M
M
Mattaj
I W
IW
D000595:Amino Acid Sequence; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002451:Cell Compartmentation / Q000502:physiology; D006160:Guanosine Triphosphate / Q000378:metabolism; D066298:In Vitro Techniques; D028884:Karyopherins; D008969:Molecular Sequence Data; D009685:Nuclear Envelope / Q000235:genetics* / Q000378:metabolism*; D029543:Nucleocytoplasmic Transport Proteins; D009865:Oocytes / Q000502:physiology; D010750:Phosphoproteins / Q000235:genetics* / Q000378:metabolism*; D010766:Phosphorylation; D012342:RNA, Small Nuclear / Q000235:genetics* / Q000378:metabolism*; D016601:RNA-Binding Proteins; D011817:Rabbits; D018160:Receptors, Cytoplasmic and Nuclear; D012156:Reticulocytes; D014981:Xenopus; D020931:ran GTP-Binding Protein / Q000378:metabolism; D000097606:Exportin 1 Protein
D002352:Carrier Proteins; D006160:Guanosine Triphosphate; D028884:Karyopherins; D029543:Nucleocytoplasmic Transport Proteins; D010750:Phosphoproteins; D012342:RNA, Small Nuclear; D016601:RNA-Binding Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D020931:ran GTP-Binding Protein; D000097606:Exportin 1 Protein; C407263:Phax protein, mouse