Nup1p and Nsp1p are structurally related nuclear pore complex (NPC) proteins which contain many degenerate repeat sequences of the FSFG type in their central domains. To find out whether this similarity also reflects a functional overlap at the NPC, we analyzed delta nup1 cells in comparison to ts nsp1 cells for defects in nucleocytoplasmic transport. When the NUP1 gene was disrupted in two different laboratory yeast strains, haploid delta nup1 progeny was viable, showing that NUP1 is not essential for vegetative cell growth; delta nup1 strains, however, exhibited a strongly reduced growth rate at 37 degrees C as compared to 23 degrees C. When analyzed by thin section electron microscopy, delta nup1 cells show a normal nuclear envelope morphology and the number and appearance of nuclear pore complexes apparently was not altered. Whereas delta nup1 cells grown at 37 degrees C could still accumulate a lacZ reporter protein carrying a nuclear localization sequence (NLS) inside the nucleus, poly(A)+ RNA export was significantly inhibited. Our data suggest that Nup1p which is not physically associated with Nsp1p, is required for efficient nucleocytoplasmic transport reactions.