The yeast mRNA-binding protein Npl3p interacts with the cap-binding complex

A number of RNA-binding proteins are associated with mRNAs in both the nucleus and the cytoplasm. One of these, Np13p, is a heterogeneous nuclear ribonucleoprotein-like protein with some similarity to SR proteins and is essential for growth in the yeast S. cerevisiae. Temperature-sensitive alleles have defects in the export of mRNA out of the nucleus (1). In this report, we define a genetic relationship between NPL3 and the nonessential genes encoding the subunits of the cap-binding complex (CBP80 and CBP20). Deletion of CBP80 or CBP20 in combination with certain temperature-sensitive np13 mutant alleles fail to grow and thus display a synthetic lethal relationship. Further evidence of an interaction between Np13p and the cap-binding complex was revealed by co-immunoprecipitation experiments; Cbp80p and Cbp20p specifically co-precipitate with Np13p. However, the interaction of Np13p with Cbp80p depends on both the presence of Cbp20p and RNA. In addition, we show that Cbp80p is capable of shuttling between the nucleus and the cytoplasm in a manner dependent on the ongoing synthesis of RNA. Taken together, these data support a model whereby mRNAs are co-transcriptionally packaged by proteins including Np13p and cap-binding complex for export out of the nucleus.