A Crm1p-independent nuclear export path for the mRNA-associated protein, Npl3p/Mtr13p

mRNA export involves association of mRNAs with nucleoplasmic proteins, deli very to the nuclear pore complex, translocation to the cytoplasm, and reimp ort of recycling components. Many yeast mutants inhibit mRNA export, but th ere is little information concerning the RNA carriers and steps of transpor t that they affect. The hnRNP/serine-arginine-rich-like protein, Np13p/Mtr1 3p, binds poly(A)(+) RNA and shuttles between the nucleus and cytoplasm, It s export accelerates on inhibition of RNA synthesis. In vivo tests show tha t its export requires two proteins with putative leucine-rich nuclear expor t signals: Gle1p, Mex67p, and several additional nuclear and nuclear pore c omplex-associated proteins. Surprisingly, a nonnuclear pool of an import fa ctor (the importin alpha homologue, Srp1p) is also required. Changes in the methylation status of Np13p do not correlate with its nucleocytoplasmic di stribution. A crm1 mutant that inhibits export of proteins with leucine-ric h nuclear export signals and mRNAs does not inhibit Np13p export, Moreover, several proteins needed for Np13p export are not needed for export of a ty pical Crm1p cargo. Thus, Np13p export requires only a subset of proteins im plicated in mRNA export, suggesting that more than one mRNA export path exi sts, A distinct group of mutants, including a mutation of a member of the i mportin beta superfamily, inhibits Np13p reimport from the cytoplasm.