Many messenger RNA export proteins have been identified; yet the spatial and temporal activities of these proteins and how they determine directionality of messenger ribonucleoprotein (mRNP) complex export from the nucleus remain largely undefined. Here, the bacteriophage PP7 RNA-labeling system was used in Saccharomyces cerevisiae to follow single-particle mRNP export events with high spatial precision and temporal resolution. These data reveal that mRNP export, consisting of nuclear docking, transport, and cytoplasmic release from a nuclear pore complex (NPC), is fast ( approximately 200 ms) and that upon arrival in the cytoplasm, mRNPs are frequently confined near the nuclear envelope. Mex67p functions as the principal mRNP export receptor in budding yeast. In a mex67-5 mutant, delayed cytoplasmic release from NPCs and retrograde transport of mRNPs was observed. This proves an essential role for Mex67p in cytoplasmic mRNP release and directionality of transport.

Autoregulatory loops often provide precise control of the level of expression of specific genes that encode key regulatory proteins. Here we have defined a pathway by which Yra1p, a yeast mRNA export factor, controls its own expression. We show that YRA1 exon 1 sequences in cis and Yra1p in trans inhibit YRA1 pre-mRNA splicing and commit the pre-mRNA to nuclear export. Mex67p and Crm1p jointly promote YRA1 pre-mRNA export, and once in the cytoplasm, the pre-mRNA is degraded by a 5' to 3' decay mechanism that is dependent on the decapping activator Edc3p and on specific sequences in the YRA1 intron. These results illustrate how common steps in the nuclear processing, export, and degradation of a transcript can be uniquely combined to control the expression of a specific gene and suggest that Edc3p-mediated decay may have additional regulatory functions in eukaryotic cells.

Localization of beta-actin messenger RNA to sites of active actin polymerization modulates cell migration during embryogenesis, differentiation and possibly carcinogenesis. This localization requires the oncofetal protein ZBP1 (Zipcode binding protein 1), which binds to a conserved 54-nucleotide element in the 3'-untranslated region of the beta-actin mRNA known as the 'zipcode'. ZBP1 promotes translocation of the beta-actin transcript to actin-rich protrusions in primary fibroblasts and neurons. It is not known how the ZBP1-RNA complex achieves asymmetric protein sorting by localizing beta-actin mRNA. Here we show that chicken ZBP1 modulates the translation of beta-actin mRNA. ZBP1 associates with the beta-actin transcript in the nucleus and prevents premature translation in the cytoplasm by blocking translation initiation. Translation only occurs when the ZBP1-RNA complex reaches its destination at the periphery of the cell. At the endpoint of mRNA transport, the protein kinase Src promotes translation by phosphorylating a key tyrosine residue in ZBP1 that is required for binding to RNA. These sequential events provide both temporal and spatial control over beta-actin mRNA translation, which is important for cell migration and neurite outgrowth.