TARGETING AND FUNCTION IN MESSENGER-RNA EXPORT OF NUCLEAR-PORE COMPLEX PROTEIN NUP153

Nup153 is a large (153 kD) O-linked glycoprotein which is a component of the basket structure located on the nucleoplasmic face of nuclear p ore complexes. This protein exhibits a tripartite structure consisting of a zinc finger domain flanked by large (60-70 kD) NH2- and COOH-ter minal domains. When full-length human Nup153 is expressed in BHK cells , it accumulates appropriately at the nucleoplasmic face of the nuclea r envelope. Targeting information for Nup153 resides in the NH2-termin al domain since this region of the molecule can direct an ordinarily c ytoplasmic protein, pyruvate kinase, to the nuclear face of the nuclea r pore complex. Overexpression of Nup153 results in the dramatic accum ulation of nuclear poly (A)(+) RNA, suggesting an inhibition of RNA ex port from the nucleus. This is not due to a general decline in nucleoc ytoplasmic transport or to occlusion or loss of nuclear pore complexes since nuclear protein import is unaffected. While overexpression of c ertain Nup153 constructs was found to result in the formation of unusu al intranuclear membrane arrays, this structural phenotype could not b e correlated with the effects on poly (A)(+) RNA distribution, The RNA trafficking defect was, however, dependent upon the Nup153 COOH-termi nal domain which contains most of the XFXFG repeats. It is proposed th at this region of Nup153, lying within the distal ring of the nuclear basket, represents a docking site for mRNA molecules exiting the nucle us.