Phosphorylation and Dephosphorylation of SR Proteins Imposes Directionality on mRNP Export Across the Nuclear Pore Complex
Studies of S. cerevisiae indicate that the direction of mRNP export from the nucleus into the cytoplasm is controlled by the phosphorylation and dephosphorylation of mRNP adapter proteins, such as REF, that assist in the binding of the NXF1/NXT1 mRNP exporter to mRNPs. In one case, a yeast SR protein (Npl3) functions as an adapter protein that promotes the binding of the yeast mRNP exporter (Figure 10-24). In its phosphorylated form, the SR protein initially binds to nascent pre-mRNA. When 3′ cleavage and polyadenylation are completed, the adapter protein is dephosphorylated by a specific nuclear protein phosphatase that is essential for mRNP export. Only the dephosphorylated adapter protein can bind the mRNP exporter, thereby coupling mRNP export to correct polyadenylation. This mechanism is one form of mRNA “quality control.” If the nascent mRNP is not correctly processed, it is not recognized by the phosphatase that dephosphorylates Npl3, and consequently, it is not bound by the mRNP exporter and is not exported from the nucleus. Instead, it is degraded by exosomes, the multiprotein complexes that degrade unprotected RNAs in the nucleus and cytoplasm (see Figures 10-1 and 10-16).
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.
Following export to the cytoplasm, the Npl3 SR protein is phosphorylated by a specific cytoplasmic protein kinase. This phosphorylation causes it to dissociate from the mRNP, along with the mRNP exporter. In this way, dephosphorylation of mRNP adapter proteins in the nucleus once RNA processing is complete and their phosphorylation and resulting dissociation in the cytoplasm result in a higher concentration of mRNP exporter–mRNP complexes in the nucleus, where they form, and a lower concentration of these complexes in the cytoplasm, where they dissociate. As a result, the direction of mRNP export may be driven by simple diffusion down a concentration gradient of the mRNP exporter–mRNP complex across the NPC, from high in the nucleus to low in the cytoplasm.