Kinetic impairment of restrictive streptomycin-resistant ribosomes

Molecular Genetics and Genomics - Comparisons in vivo and in vitro of wild-type and otherwise isogenic bacteria with five different mutant alleles of the gene (rpsL) specifying ribosomal protein...     

Isolation and characterisation of DOCK8, a member of the DOCK180-related regulators of cell morphology

In a yeast two-hybrid system screen for Cdc42-interacting proteins, we identified a protein with similarity to the CrkII-binding protein DOCK180. A cDNA clone of this protein, designated DOCK8, encoded a gene-product of 1701 amino acid residues with a molecular mass of 190 kDa. Immunofluorescence staining showed that transiently transfected HA-tagged DOCK8, as well as endogenous DOCK8, was present at the cell edges in areas undergoing lamellipodia formation. Transient transfection of a C-terminal fragment of DOCK8 resulted in the formation of vesicular structures. Interestingly, these vesicles also contained filamentous actin. These data suggest an involvement of DOCK8 in processes that affect the organisation of filamentous actin.     

The Activity of Hyaluronan Synthase 2 Is Regulated by Dimerization and Ubiquitination

Hyaluronan is a component of the extracellular matrix, which affects tissue homeostasis. In this study, we investigated the regulatory mechanisms of one of the hyaluronan-synthesizing enzymes, HAS2. Ectopic expression of Flag- and 6myc-HAS2 in COS-1 cells followed by immunoprecipitation and immunoblotting revealed homodimers; after co-transfection with Flag-HAS3, also heterodimers were seen. Furthermore, the expressed HAS2 was ubiquitinated. We identified one acceptor site for ubiquitin on lysine residue 190. Mutation of this residue led to inactivation of the enzymatic activity of HAS2. Interestingly, K190R-mutated HAS2 formed dimers with wt HAS2 and quenched the activity of wt HAS2, thus demonstrating a functional role of the dimeric configuration.     

Mitogenic response of human SH-SY5Y neuroblastoma cells to insulin-like growth factor I and II is dependent on the stage of differentiation

Human insulin-like growth factor I and II (IGF-I and IGF-II) in concentrations of 1-30 ng/ml, were shown to stimulate ornithine decarboxylase activity and [3H]thymidine incorporation in human SH-SY5Y neuroblastoma cells. Proliferation of these cells was also stimulated by IGF-I and II when added to RPMI 1640 medium, fortified with selenium, hydrocortisone, transferrin, and beta-estradiol. Labeled IGF-I and II bound to SH-SY5Y cells. The cross-reaction pattern of IGF-I, IGF-II, and insulin in competing with the binding of labeled IGF-I and IGF-II, respectively, indicated that SH-SY5Y cells express both type I and type II IGF receptors. Treatment of SH-SY5Y cells for 4 d with 12-O-tetradecanoylphorbol-13-acetate (TPA), which resulted in morphological and functional differentiation and growth inhibition, abolished the mitogenic response to both IGF-I and II. Concomitantly, the binding of IGF-II disappeared almost totally, which offers a possible explanation for the reduced biological response to IGF-II after TPA treatment. In contrast, the IGF-I binding in TPA-treated cells was only reduced to approximately 70% of the binding to control cells. It is therefore not excluded that the IGF-I receptor could be uncoupled by TPA, with persistent binding capacity for IGF-I.     

Dissociation and reassociation of oligomeric viral glycoprotein subunits in the endoplasmic reticulum.

The vesicular stomatitis virus (VSV) glycoprotein (G) forms noncovalently linked trimers in the endoplasmic reticulum (ER) prior to transport to the cell surface. Here we examined the formation of heterotrimers between wild-type and mutant subunits that were retained in the ER by C-terminal retention signals. When G protein was coexpressed with mutant subunits that formed trimers at the wild-type rate and were transported from the ER at the wild-type rate, heterotrimers were readily detected. In contrast, when G protein was coexpressed with mutant subunits that formed trimers at the wild-type rate, but were retained in the ER, heterotrimers were not detected unless transport of the wild-type molecules from the ER was blocked. After removal of transport block, the heterotrimers then dissociated and reassorted to homotrimers of the mutant protein that were retained in the ER and wild-type trimers that were transported to the cell surface. These and other results presented here indicate that there is an equilibrium between G protein trimers and monomers in vivo, at least in the ER. This equilibrium may function to allow escape of wild-type subunits from aberrant retained subunits.     

Changes in inducibility of ornithine decarboxylase activity in differentiating human neuroblastoma cells

Human SH-SY5Y neuroblastoma cells could be induced to differentiate morphologically and biochemically in the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA), retinoic acid (RA), or a combination of these two substances. The phenotypical changes induced by these substances differed, but one effect of both was an inhibition of the cell growth. Addition of TPA or RA to non-treated cells had no effect on the activation of ornithine decarboxylase (ODC, EC 4.1.1.17.), while a change to fresh medium stimulated the ODC to maximum activity after 4-6 h. The activity was not altered by the presence of RA in the fresh medium, but TPA partially inhibited the medium-stimulated ODC activity. Cells treated for 4 or 8 days with TPA or a combination of TPA and RA had a low ODC activity which could not be induced by fresh medium. However, RA-treated (and thus growth-inhibited) cells still responded to a change of medium by exhibiting an ODC activity of the same magnitude and duration as in medium-stimulated control cells. The results seem to suggest that the growth inhibition induced by TPA and RA, respectively, is mediated by different mechanisms.     

The atypical Rho GTPases Miro-1 and Miro-2 have essential roles in mitochondrial trafficking

We recently described the atypical Rho GTPases Miro-1 and Miro-2. These proteins have tandem GTP-binding domains separated by a linker region with putative calcium-binding motives. In addition, the Miro GTPases have a C-terminal transmembrane domain, which confers targeting to the mitochondria. It was reported previously that a constitutively active mutant of Miro-1 induced a clustering of the mitochondria. This response can be separated into two distinct phenotypes: a formation of aggregated mitochondria and the appearance of thread-like mitochondria probably caused by defects in mitochondrial trafficking. The first GTPase domain is required for the clustering of the mitochondria, but the effect is not dependent on the EF-hands. Miro-2 only induces aggregation and not the formation of thread-like mitochondria. Moreover, we show that Miro interacts with the Kinesin-binding proteins, GRIF-1 and OIP106, suggesting that the Miro GTPases form a link between the mitochondria and the trafficking apparatus of the microtubules.     

Effects of altered cytoplasmic domains on transport of the vesicular stomatitis virus glycoprotein are transferable to other proteins.

Alterations of the cytoplasmic domain of the vesicular stomatitis virus glycoprotein (G protein) were shown previously to affect transport of the protein from the endoplasmic reticulum, and recent studies have shown that this occurs without detectable effects on G protein folding and trimerization (R. W. Doms et al., J. Cell Biol., in press). Deletions within this domain slowed exit of the mutant proteins from the endoplasmic reticulum, and replacement of this domain with a foreign 12-amino-acid sequence blocked all transport out of the endoplasmic reticulum. To extend these studies, we determined whether such effects of cytoplasmic domain changes were transferable to other proteins. Three different assays showed that the effects of the mutations on transport of two membrane-anchored secretory proteins were the same as those observed with vesicular stomatitis virus G protein. In addition, possible effects on oligomerization were examined for both transported and nontransported forms of membrane-anchored human chorionic gonadotropin-alpha. These membrane-anchored forms, like the nonanchored human chorionic gonadotropin-alpha, had sedimentation coefficients consistent with a monomeric structure. Taken together, our results provide strong evidence that these cytoplasmic mutations affect transport by affecting interactions at or near the cytoplasmic side of the membrane.     

The atypical Rho GTPase Wrch1 collaborates with the nonreceptor tyrosine kinases Pyk2 and Src in regulating cytoskeletal dynamics

The Cdc42-like GTPase Wnt responsive Cdc42 homolog 1 (Wrch1) has several atypical features; it has an N-terminal proline-rich extension that confers binding to SH3 domains, and it harbors an extremely high intrinsic nucleotide exchange activity, which overrides the normal GTPase activity. As a result, Wrch1 resides mainly in the active, GTP-loaded conformation under normal cellular conditions. We have previously shown that ectopic expression of Wrch1 in fibroblasts resulted in an altered cell morphology visible as a formation of filopodia, a loss of stress fibers, and a reduction in focal adhesions. Here, we show that Wrch1 binds to the nonreceptor tyrosine kinase Pyk2. The interaction required Wrch1 to be in a GTP conformation and also required an intact N-terminal proline-rich extension as well as an intact effector loop. Wrch1 requires Pyk2 in imposing the cytoskeletal effects, seen as the formation of filopodia, since treatment of cells with a Pyk2-specific small interfering RNA abrogated this response. Interestingly, we found that the presence and activity of Src were needed for the formation of a Wrch1-Pyk2 complex as well as for the Wrch1-induced formation of filopodia. We propose a model in which Pyk2 and Src function to coordinate the Wrch1-dependent effects on cytoskeletal dynamics.