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 nucl eus and the cytoplasm. One of these, Np13p, is a heterogeneous nuclear ribo nucleoprotein-like protein with some similarity to SR proteins and is essen tial for growth in the yeast S. cerevisiae. Temperature-sensitive alleles h ave defects in the export of mRNA out of the nucleus (1). In this report, w e define a genetic relationship between NPL3 and the nonessential genes enc oding the subunits of the cap-binding complex (CBP80 and CBP20). Deletion o f CBP80 or CBP20 in combination with certain temperature-sensitive np13 mut ant alleles fail to grow and thus display a synthetic lethal relationship. Further evidence of an interaction between Np13p and the cap-binding comple x was revealed by co-immunoprecipitation experiments; Cbp80p and Cbp20p spe cifically co-precipitate with Np13p. However, the interaction of Np13p with Cbp80p depends on both the presence of Cbp20p and RNA. In addition, we sho w that Cbp80p is capable of shuttling between the nucleus and the cytoplasm in a manner dependent on the ongoing synthesis of RNA. Taken together, the se data support a model whereby mRNAs are co-transcriptionally packaged by proteins including Np13p and cap-binding complex for export out of the nucl eus.