Mutational Analyses of the Cysteine-Rich Domain of Yvh1, a Protein Required for Translational Competency in Yeast.

Ribosome assembly is a complex biological process facilitated by >200 trans-acting factors (TAFs) that function as scaffolds, place-holders or complex remodelers to promote efficient and directional ribosomal subunit assembly but are not themselves part of functional ribosomes. One such yeast TAF is encoded by Mrt4 which assembles onto pre-60S complexes in the nuclear compartment and remains bound to pre-60S complexes as they are exported into the cytoplasm. There, Mrt4 is displaced from pre-60S complexes facilitating the subsequent addition of the ribosomal stalk complex (P0/P1/P2). Ribosomal stalk proteins interact with translational GTPases (trGTPase) which facilitate and control protein synthesis on the ribosome. The rRNA-binding domain of Mrt4 is structurally similar to P0, with both proteins binding to the same interface of pre-60S subunits in a mutually exclusive manner; the addition of the ribosomal stalk therefore requires the displacement of Mrt4 from pre-60S subunits. Mrt4 removal requires the C-terminal cysteine-rich domain (CRD) of the dual-specificity phosphatase Yvh1. Unlike many other TAFs, yeast lacking Yvh1 are viable but retain Mrt4 on cytoplasmic pre-60S complexes precluding ribosomal stalk addition. Although Yvh1's role in Mrt4 removal is well established, how Yvh1 accomplishes this is largely unknown. Here, we report an unbiased genetic screen to isolate Yvh1 variants that fail to displace Mrt4 from pre-60S ribosomes. Bioorthogonal non-canonical amino acid tagging (BONCAT) approaches demonstrate that these YVH1 loss-of-function variants also display defects in nascent protein production. The further characterization of one LOF variant, Yvh1F283L, establishes it as an expression-dependent, dominant-negative variant capable of interfering with endogenous Yvh1 function, and we describe how this Yvh1 variant can be used as a novel probe to better understand ribosome maturation and potentially ribosome heterogeneity in eukaryotes.

Overexpression of eIF5 or its protein mimic 5MP perturbs eIF2 function and induces ATF4 translation through delayed re-initiation.

ATF4 is a pro-oncogenic transcription factor whose translation is activated by eIF2 phosphorylation through delayed re-initiation involving two uORFs in the mRNA leader. However, in yeast, the effect of eIF2 phosphorylation can be mimicked by eIF5 overexpression, which turns eIF5 into translational inhibitor, thereby promoting translation of GCN4, the yeast ATF4 equivalent. Furthermore, regulatory protein termed eIF5-mimic protein (5MP) can bind eIF2 and inhibit general translation. Here, we show that 5MP1 overexpression in human cells leads to strong formation of 5MP1:eIF2 complex, nearly comparable to that of eIF5:eIF2 complex produced by eIF5 overexpression. Overexpression of eIF5, 5MP1 and 5MP2, the second human paralog, promotes ATF4 expression in certain types of human cells including fibrosarcoma. 5MP overexpression also induces ATF4 expression in Drosophila The knockdown of 5MP1 in fibrosarcoma attenuates ATF4 expression and its tumor formation on nude mice. Since 5MP2 is overproduced in salivary mucoepidermoid carcinoma, we propose that overexpression of eIF5 and 5MP induces translation of ATF4 and potentially other genes with uORFs in their mRNA leaders through delayed re-initiation, thereby enhancing the survival of normal and cancer cells under stress conditions.

Characterization of a human cell line stably over-expressing the candidate oncogene, dual specificity phosphatase 12.

BACKGROUND: Analysis of chromosomal rearrangements within primary tumors has been influential in the identification of novel oncogenes. Identification of the "driver" gene(s) within cancer-derived amplicons is, however, hampered by the fact that most amplicons contain many gene products. Amplification of 1q21-1q23 is strongly associated with liposarcomas and microarray-based comparative genomic hybridization narrowed down the likely candidate oncogenes to two: the activating transcription factor 6 (atf6) and the dual specificity phosphatase 12 (dusp12). While atf6 is an established transcriptional regulator of the unfolded protein response, the potential role of dusp12 in cancer remains uncharacterized. METHODOLOGY/PRINCIPAL FINDINGS: To evaluate the oncogenic potential of dusp12, we established stable cell lines that ectopically over-express dusp12 in isolation and determined whether this cell line acquired properties frequently associated with transformed cells. Here, we demonstrate that cells over-expressing dusp12 display increased cell motility and resistance to apoptosis. Additionally, over-expression of dusp12 promoted increased expression of the c-met proto-oncogene and the collagen and laminin receptor intergrin alpha 1 (itga1) which is implicated in metastasis. SIGNIFICANCE: Collectively, these results suggest that dusp12 is oncologically relevant and exposes a potential association between dusp12 and established oncogenes that could be therapeutically targeted.

Role of group A p21-activated kinases in activation of extracellular-regulated kinase by growth factors.

The canonical extracellular-regulated kinase (ERK) signaling cascade, consisting of the Ras-Raf-Mek-ERK module, is critically important to many cellular functions. Although the general mechanism of activation of the ERK cascade is well established, additional noncanonical components greatly influence the activity of this pathway. Here, we focus on the group A p21-activated kinases (Paks), which have previously been implicated in regulating both c-Raf and Mek1 activity, by phosphorylating these proteins at Ser(338) and Ser(298), respectively. In NIH-3T3 cells, expression of an inhibitor of all three group A Paks reduced activation of ERK in response to platelet-derived growth factor (PDGF) but not to epidermal growth factor (EGF). Similar results were obtained in HeLa cells using small interference RNA-mediated simultaneous knockdown of both Pak1 and Pak2 to reduce group A Pak function. Inhibition of Pak kinase activity dramatically decreased phosphorylation of Mek1 at Ser(298) in response to either PDGF or EGF, but this inhibition did not prevent Mek1 activation by EGF, suggesting that although Pak can phosphorylate Mek1 at Ser(298), this event is not required for Mek1 activation by growth factors. Inhibition of Pak reduced the Ser(338) phosphorylation of c-Raf in response to both PDGF and EGF; however, in the case of EGF, the reduction in Ser(338) phosphorylation was not accompanied by a significant decrease in c-Raf activity. These findings suggest that Paks are required for the phosphorylation of c-Raf at Ser(338) in response to either growth factor, but that the mechanisms by which EGF and PDGF activate c-Raf are fundamentally different.

Vav1 transduces T cell receptor signals to the activation of the Ras/ERK pathway via LAT, Sos, and RasGRP1.

Vav1 is a signaling protein required for both positive and negative selection of CD4(+)CD8(+) double positive thymocytes. Activation of the ERK MAPK pathway is also required for positive selection. Previous work has shown that Vav1 transduces T cell receptor (TCR) signals leading to an intracellular calcium flux. We now show that in double positive thymocytes Vav1 is required for TCR-induced activation of the ERK1 and ERK2 kinases via a pathway involving the Ras GTPase, and B-Raf, MEK1, and MEK2 kinases. Furthermore, we show that Vav1 transduces TCR signals to Ras by controlling the membrane recruitment of two guanine nucleotide exchange factors. First, Vav1 transduces signals via phospholipase Cgamma1 leading to the membrane recruitment of RasGRP1. Second, Vav1 is required for recruitment of Sos1 and -2 to the transmembrane adapter protein LAT. Finally, we show that Vav1 is required for TCR-induced LAT phosphorylation, a key event for the activation of both phospholipase Cgamma1 and Sos1/2. We propose that reduced LAT phosphorylation is the key reason for defective TCR-induced calcium flux and ERK activation in Vav1-deficient cells.

p21-Activated kinase 5 (Pak5) localizes to mitochondria and inhibits apoptosis by phosphorylating BAD.

Pak5 is the most recently identified and least understood member of the p21-activated kinase (Pak) family. This kinase is known to promote neurite outgrowth in vitro, but its localization, substrates, and effects on cell survival have not been reported. We show here that Pak5 has unique properties that distinguish it from all other members of the Pak family. First, Pak5, unlike Pak1, cannot complement an STE20 mutation in Saccharomyces cerevisiae. Second, Pak5 binds to the GTPases Cdc42 and Rac, but these GTPases do not regulate Pak5 kinase activity, which is constitutive and stronger than any other Pak. Third, Pak5 prevents apoptosis induced by camptothecin and C2-ceramide by phosphorylating BAD on Ser-112 in a protein kinase A-independent manner and prevents the localization of BAD to mitochondria, thereby inhibiting the apoptotic cascade that leads to apoptosis. Finally, we show that Pak5 itself is constitutively localized to mitochondria, and that this localization is independent of kinase activity or Cdc42 binding. These features make Pak5 unique among the Pak family and suggest that it plays an important role in apoptosis through BAD phosphorylation.

p21-activated kinase links Rac/Cdc42 signaling to merlin.

The neurofibromatosis type 2 tumor suppressor gene, NF2, is mutated in the germ line of NF2 patients and predisposes affected individuals to intracranial and spinal tumors. Moreover, somatic mutations of NF2 can occur in the sporadic counterparts of these neurological tumor types as well as in certain neoplasms of non-neuroectodermal origin, such as malignant mesothelioma and melanoma. NF2 encodes a 595-amino acid protein, merlin, which exhibits significant homology to the ezrin-radixin-moesin family of proteins. However, the mechanism by which merlin exerts its tumor suppressor activity is not well understood. In this investigation, we show that merlin is phosphorylated in response to expression of activated Rac and activated Cdc42 in mammalian cells. Furthermore, we demonstrate that merlin phosphorylation is mediated by p21-activated kinase (Pak), a common downstream target of both Rac and Cdc42. Both in vivo and in vitro kinase assays demonstrated that Pak can directly phosphorylate merlin at serine 518, a site that affects merlin activity and localization. These biochemical investigations provide insights into the regulation of merlin function and establish a framework for elucidating tumorigenic mechanisms involved in neoplasms associated with merlin inactivation.