The Ser-Arg-rich (SR) proteins comprise a large family of nuclear phosphoproteins that are required for constitutive and alternative splicing. A subset of SR proteins shuttles continuously between the nucleus and the cytoplasm, suggesting that the role of shuttling SR proteins in gene expression may not be limited to nuclear pre-mRNA splicing, but may also include unknown cytoplasmic functions. Here, we show that shuttling SR proteins, in particular SF2/ASF, associate with translating ribosomes and stimulate translation when tethered to a reporter mRNA in Xenopus oocytes. Moreover, SF2/ASF enhances translation of reporter mRNAs in HeLa cells, and this activity is dependent on its ability to shuttle from the nucleus to the cytoplasm and is increased by the presence of an exonic-splicing enhancer. Furthermore, SF2/ASF can stimulate translation in vitro using a HeLa cell-free translation system. Thus, the association of SR proteins with translating ribosomes, as well as the stimulation of translation both in vivo and in vitro, strongly suggest a role for shuttling SR proteins in translation. We propose that shuttling SR proteins play multiple roles in the posttranscriptional expression of eukaryotic genes and illustrate how they may couple splicing and translation.[Keywords: pre-mRNA splicing; SR proteins; nucleocytoplasmic shuttling; ribosomes; translation] Supplemental material is available at http://www.genesdev.org. Pre-mRNA splicing, an essential step in gene expression, is catalyzed by a large ribonucleoprotein complex, termed the spliceosome. This macromolecular machine consists of small nuclear ribonucleoproteins particles (snRNPs) U1, U2, U4, U5, and U6 and a multitude of non-snRNP splicing factors that includes Ser-Arg-rich (SR) proteins (for review, see Kramer 1996;Will and Luhrmann 1997;Jurica and Moore 2003). The SR proteins constitute a family of structurally and functionally related proteins, playing dual roles in both constitutive and alternative pre-mRNA splicing (Fu 1995;Valcarcel and Green 1996;Graveley 2000). SR proteins are characterized by their modular domain structure, with one or two N-terminal RNA recognition motifs (RRMs) and a C-terminal domain rich in arginine and serine residues, termed the RS domain. The RRMs determine RNA-binding specificity, whereas the RS domain acts as a splicing activator domain by mediating protein-protein interactions with other components of the splicing machinery (Wu and Maniatis 1993; Graveley and Maniatis 1998). The RS domain also directs subcellular localization and nucleocytoplasmic shuttling of individual SR proteins (Hedley et al. 1995;Caceres et al. 1997Caceres et al. , 1998.SR proteins play numerous roles in pre-mRNA splicing and spliceosome assembly, but perhaps their most significant function is in splice-site recognition and selection (Tacke and Manley 1999;Sanford et al. 2003). Metazoan splice sites contain low information content and are far more degenerate than those found in simpler eukaryotes (Burge et al. 1998); thus, splice-site specificity ...

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Atherosclerotic lesions are asymmetric focal thickenings of the intima of arteries that consist of lipids, various cell types and extracellular matrix (ECM). These lesions lead to vascular occlusion representing the most common cause of death in the Western world. The main cause of vascular occlusion is rupture of atheromatous lesions followed by thrombus formation. Fibronectin (FN) is one of the earliest ECM proteins deposited at atherosclerosis-prone sites and was suggested to promote atherosclerotic lesion formation. Here, we report that atherosclerosis-prone apolipoprotein E-null mice lacking hepatocyte-derived plasma FN (pFN) fed with a pro-atherogenic diet display dramatically reduced FN depositions at atherosclerosis-prone areas, which results in significantly smaller and fewer atherosclerotic plaques. However, the atherosclerotic lesions from pFN-deficient mice lacked vascular smooth muscle cells and failed to develop a fibrous cap. Thus, our results demonstrate that while FN worsens the course of atherosclerosis by increasing the atherogenic plaque area, it promotes the formation of the protective fibrous cap, which in humans prevents plaques rupture and vascular occlusion.

MSL-2 (male-specific lethal 2) is the limiting component of the Drosophila dosage compensation complex (DCC) that specifically increases transcription from the male X chromosome. Ectopic expression of MSL-2 protein in females causes DCC assembly on both X chromosomes and lethality. Inhibition of MSL-2 synthesis requires the female-specific protein sex-lethal (SXL), which binds to the msl-2 mRNA 5 and 3 untranslated regions (UTRs) and blocks translation through distinct UTR-specific mechanisms. Here, we purify translationally silenced msl-2 mRNPs and identify UNR (upstream of N-ras) as a protein recruited to the 3 UTR by SXL. We demonstrate that SXL requires UNR as a corepressor for 3-UTR-mediated regulation, imparting a female-specific function to the ubiquitously expressed UNR protein. Our results reveal a novel functional role for UNR as a translational repressor and indicate that UNR is a key component of a "fail-safe" dosage compensation regulatory system that prevents toxic MSL-2 synthesis in female cells.[Keywords: Translational control; dosage compensation; mRNP; cold shock domain; female cell line; sex-specific function] Supplemental material is available at http://www.genesdev.org.

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