An essential yeast protein, Nup145p, is identified via its genetic interaction with the nucleoporin Nsp1p. Nup145p contains GLFG repeats and localizes to nuclear pores. Depletion of Nup145p in vivo leads rapidly to nuclear retention of polyadenylated RNAs and more slowly to cytoplasmic accumulation of a nuclear reporter protein. A stretch of 140 amino acids within Nup145p is conserved in two other yeast nucleoporins, Nup116p and Nup100p, and in an uncharacterized C. elegans protein. Genetic experiments in yeast reveal that the three copies of the motif carry out an essential, redundant function. Fragments of Nup145p and Nup116p including this motif bind specifically to homopolymeric RNAs in vitro. Nup145p, Nup116p, and Nup100p thus represent a novel class of nucleoporins involved in nucleocytoplasmic transport.