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Publications (3)11.82 Total impact

  • Bradley Ford · Karlheinz Skowronek · Sean Boykevisch · Dafna Bar-Sagi · Nicolas Nassar
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    ABSTRACT: Substituting alanine for glycine at position 60 in v-H-Ras generated a dominant negative mutant that completely abolished the ability of v-H-Ras to transform NIH 3T3 cells and to induce germinal vesicle breakdown in Xenopus oocytes. The crystal structure of the GppNp-bound form of RasG60A unexpectedly shows that the switch regions adopt an open conformation reminiscent of the structure of the nucleotide-free form of Ras in complex with Sos. Critical residues that normally stabilize the guanine nucleotide and the Mg(2+) ion have moved considerably. Sos binds to RasG60A but is unable to catalyze nucleotide exchange. Our data suggest that the dominant negative effect observed for RasG60A.GTP could result from the sequestering of Sos in a non-productive Ras-GTP-guanine nucleotide exchange factor ternary complex.
    Journal of Biological Chemistry 08/2005; 280(27):25697-705. DOI:10.1074/jbc.M502240200 · 4.57 Impact Factor
  • Karlheinz R Skowronek · Fukun Guo · Yi Zheng · Nicolas Nassar
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    ABSTRACT: The multidomain protein Trio regulates among others neuronal outgrowth and axonal guidance in vertebrates and invertebrates. Trio contains two Dbl-homology/pleckstrin homology (DH/PH) tandem domains that activate several RhoGTPases. Here, we present the x-ray structure of the N-terminal DH/PH, hereafter TrioN, refined to 1.7-A resolution. We show that the relative orientations of the DH and PH domains of TrioN and free Dbs are similar. However, this relative orientation is dissimilar to Dbs in the Dbs/Cdc42 structure. In vitro nucleotide exchange experiments catalyzed by TrioN show that RhoG is approximately 3x more efficiently exchanged than Rac and support the conclusion that RhoG is likely the downstream target of TrioN. Residues 54 and 69, which are not conserved between the two GTPases, are responsible for this specificity. Dot-blot assay reveals that the TrioN-PH domain does not detectably bind phosphatidylinositol 3,4-bisphosphate, PtdIns(3,4)P(2), or other phospholipids. This finding is supported by our three-dimensional structure and affinity binding experiments. Interestingly, the presence of RhoG but not Rac or a C-terminal-truncated RhoG mutant allows TrioN to bind PtdIns(3,4)P(2) with a micromolar affinity constant. We conclude the variable C-terminal basic tail of RhoG specifically assists the recruitment of the TrioN-PH domain to specific membrane-bound phospholipids. Our data suggest a role for the phosphoinositide 3-kinase, PI 3-kinase, in modulating the Trio/RhoG signaling pathway.
    Journal of Biological Chemistry 10/2004; 279(36):37895-907. DOI:10.1074/jbc.M312677200 · 4.57 Impact Factor
  • Karlheinz Skowronek · Mohammed Ghumman · Yi Zheng · Nicolas Nassar
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    ABSTRACT: Trio is a multidomain signaling protein that plays an important role in neurite outgrowth, axon guidance and skeletal muscle development. Trio contains two DH/PH tandem domains that respectively activate the small GTPases RhoG/Rac and RhoA. The N-terminal DH/PH domain, TrioN, crystallizes in space group P3(1)21, with one TrioN molecule in the asymmetric unit and diffracts to 1.7 A resolution. The unit-cell parameters are a = b = 99.5, c = 98.3 A, alpha = beta = 90, gamma = 120 degrees. A greater than 90% complete native data set has been collected and structure determination using the multiple isomorphous replacement (MIR) method is ongoing.
    Acta Crystallographica Section D Biological Crystallography 08/2003; 59(Pt 7):1273-5. DOI:10.1107/S0907444903009442 · 2.67 Impact Factor