The shuttling SR protein 9G8 plays a role in translation of unspliced mRNA containing a constitutive transport element.

The splicing regulatory SR protein, 9G8, has recently been proposed to function in mRNA export in conjunction with the export protein, Tap/NXF1. Tap interacts directly with the Mason-Pfizer monkey virus constitutive transport element (CTE), an element that enables export of unspliced, intron-containing mRNA. Based on our previous finding that Tap can promote polysome association and translation of CTE-RNA, we investigated the effect of 9G8 on cytoplasmic RNA fate. 9G8 was shown to enhance expression of unspliced RNA containing either the Mason-Pfizer monkey virus-CTE or the recently discovered Tap-CTE. 9G8 also enhanced polyribosome association of unspliced RNA containing a CTE. Hyperphosphorylated 9G8 was present in monosomes and small polyribosomes, whereas soluble fractions contained only hypophosphorylated protein. Our results are consistent with a model in which hypophosphorylated SR proteins remain stably associated with messenger ribonucleoprotein (mRNP) complexes during export and are released during translation initiation concomitant with increased phosphorylation. These results provide further evidence for crucial links between RNA splicing, export and translation.

An intron with a constitutive transport element is retained in a Tap messenger RNA.

Alternative splicing is a key factor contributing to genetic diversity and evolution. Intron retention, one form of alternative splicing, is common in plants but rare in higher eukaryotes, because messenger RNAs with retained introns are subject to cellular restriction at the level of cytoplasmic export and expression. Often, retention of internal introns restricts the export of these mRNAs and makes them the targets for degradation by the cellular nonsense-mediated decay machinery if they contain premature stop codons. In fact, many of the database entries for complementary DNAs with retained introns represent them as artefacts that would not affect the proteome. Retroviruses are important model systems in studies of regulation of RNAs with retained introns, because their genomic and mRNAs contain one or more unspliced introns. For example, Mason-Pfizer monkey virus overcomes cellular restrictions by using a cis-acting RNA element known as the constitutive transport element (CTE). The CTE interacts directly with the Tap protein (also known as nuclear RNA export factor 1, encoded by NXF1), which is thought to be a principal export receptor for cellular mRNA, leading to the hypothesis that cellular mRNAs with retained introns use cellular CTE equivalents to overcome restrictions to their expression. Here we show that the Tap gene contains a functional CTE in its alternatively spliced intron 10. Tap mRNA containing this intron is exported to the cytoplasm and is present in polyribosomes. A small Tap protein is encoded by this mRNA and can be detected in human and monkey cells. Our results indicate that Tap regulates expression of its own intron-containing RNA through a CTE-mediated mechanism. Thus, CTEs are likely to be important elements that facilitate efficient expression of mammalian mRNAs with retained introns.

A revised and complete map of the chicken c-mil/raf-1 locus.

The chicken c-mil/raf-1 gene (formerly also known as c-mht) was originally identified in the search for the cellular counterpart to the v-mil oncogene of the Mill Hill 2 retrovirus and was among the first cellular proto-oncogenes discovered. Although the c-mil/raf-1 promotor, as well as the exons transduced into v-mil, were characterized in detail, an entire map of this locus has never been published. Here, we now report the location of five previously unmapped exons. In addition, we have noticed inconsistent numbering of the c-mil/raf-1 exons in the literature and the GenBank database. Thus, we provide here a complete map of the c-mil/raf-1 gene and a revision of the exon numbers. Comparison of the chicken c-mil/raf-1 gene with those of other vertebrates suggests that the numbers and lengths of the translated exons of the raf-1 locus were established early in the vertebrate lineage and have been conserved during the divergent evolution of teleosts and tetrapods.

Identification of novel ERK-mediated feedback phosphorylation sites at the C-terminus of B-Raf.

The extracellular signal-regulated kinase (ERK) pathway plays an important role during the development and activation of B lymphocytes. We have recently shown that B-Raf is a dominant ERK activator in B-cell antigen receptor signalling. We now show that B-Raf is hyperphosphorylated upon BCR engagement and undergoes a prominent electrophoretic mobility shift. This shift correlates with ERK activation and is prevented by the MEK inhibitor U0126. Syk-deficient DT40 B cells display neither dual ERK phosphorylation nor a mobility shift of B-Raf upon BCR engagement. The inducible expression of a constitutively active B-Raf in this mutant line restores dual ERK phosphorylation and the mobility shift of endogenous B-Raf, indicating that these two events are connected to each other. By site-directed mutagenesis studies, we demonstrate that the shift is due to an ERK2-mediated feedback phosphorylation of serine/threonine residues within an evolutionary conserved SPKTP motif at the C-terminus of B-Raf. Replacement of these residues by negatively charged amino acids causes a constitutive mobility shift and a reduction of PC12 cell differentiation. We discuss a model in which ERK-mediated phosphorylation of the SPKTP motif is involved in negative feedback regulation of B-Raf.

Inducible gene deletion reveals different roles for B-Raf and Raf-1 in B-cell antigen receptor signalling.

Engagement of the B-cell antigen receptor (BCR) leads to activation of the Raf-MEK-ERK pathway and Raf kinases play an important role in the modulation of ERK activity. B lymphocytes express two Raf isoforms, Raf-1 and B-Raf. Using an inducible deletion system in DT40 cells, the contribution of Raf-1 and B-Raf to BCR signalling was dissected. Loss of Raf-1 has no effect on BCR-mediated ERK activation, whereas B-Raf-deficient DT40 cells display a reduced basal ERK activity as well as a shortened BCR-mediated ERK activation. The Raf-1/B-Raf double deficient DT40 cells show an almost complete block both in ERK activation and in the induction of the immediate early gene products c-Fos and Egr-1. In contrast, BCR-mediated activation of nuclear factor of activated T cells (NFAT) relies predominantly on B-Raf. Furthermore, complementation of Raf-1/B-Raf double deficient cells with various Raf mutants demonstrates a requirement for Ras-GTP binding in BCR-mediated activation of both Raf isoforms and also reveals the important role of the S259 residue for the regulation of Raf-1. Our study shows that BCR-mediated ERK activation involves a cooperation of both B-Raf and Raf-1, which are activated specifically in a temporally distinct manner.