FIGURE 10-24 Reversible phosphorylation and direction of mRNP nuclear export. Step 1: The yeast SR protein Npl3 binds nascent pre-mRNAs in its phosphorylated form. Step 2: When polyadenylation has occurred successfully, the Glc7 nuclear phosphatase dephosphorylates Npl3, promoting the binding of the mRNP exporter, NXF1/NXT1. Step 3: The mRNP exporter allows diffusion of the mRNP complex through the central channel of the nuclear pore complex (NPC). Step 4: The cytoplasmic protein kinase Sky1 phosphorylates Npl3 in the cytoplasm, causing step 5 dissociation of the phosphorylated Npl3 from the mRNP exporter, probably through the action of an RNA helicase associated with NPC cytoplasmic filaments step 6. The mRNA transporter and phosphorylated Npl3 are transported back into the nucleus through NPCs. Step 7 Transported mRNA is available for translation in the cytoplasm. See E. Izaurralde, 2004, Nat. Struct. Mol. Biol. 11:210–212; see also W. Gilbert and C. Guthrie, 2004, Mol. Cell 13:201–212.