PubMed Health⌕ Search

Biomedical subjects

G Bossis

Publications and source records attributed to G Bossis.

6 recordsLinked to original sources
PubMed2006

Measurement of elastic forces between iron colloidal particles in a nematic liquid crystal.

The forces that arise between two iron particles in a nematic liquid crystal with a strong homeotropic anchoring were studied. For the first time, the short range repulsive force resulting from the presence of a hedgehog defect between two particles was precisely determined thanks to application of a small magnetic field and observation of the equilibrium position resulting from the balance between the elastic and magnetic forces. Above a given threshold force, the particles stuck together whereas the hedgehog defect was expelled and transformed into a Saturn ring located between the particles. The attractive part of the interparticle force was determined with the same method on the entire range of separation distances; we found that the equilibrium distance between two particles was r = 1.19 +/- 0.05 ( was the average diameter of the pair of particles).

Colloids↗
PubMed2005

A slender-body micromechanical model for viscoelasticity of magnetic colloids: comparison with preliminary experimental data.

The storage modulus, G', together with the yield stress, is an essential quantity characterizing the rheological properties of magnetic field-responsive suspensions (magnetorheological fluids or MRF). In this work, we present both experimental and theoretical results on the viscoelastic properties of MRFs. Two MRFs are used: In one the solid phase consists of cobalt ferrite particles + silica gel, with silicone oil as liquid phase. The second system is formed by carbonyl iron + silica gel also dispersed in silicone oil. The cobalt ferrite particles are synthesized as monodisperse colloidal spheres with an average diameter of 850 nm. We describe a new model based on the slender-body approach for hydrodynamic interactions. The predictions of the model are compared to preliminary experimental G' data obtained in a controlled stress plate-plate rheometer. It is found that the model gives the correct order of magnitude for the highest fields in iron suspensions, but underestimates the experimental results obtained in ferrite ones. In the case of high permeability materials such as carbonyl iron, by the inclusion of high-order multipolar interactions and saturation effects we also predict the order of magnitude of the experimental results. When dealing with low permeability cobalt ferrite based MRFs, other effects, such as remanence (at low fields) and saturation (at high fields), must be considered.

Journal Article↗
PubMed2001

Identification of a C-terminal tripeptide motif involved in the control of rapid proteasomal degradation of c-Fos proto-oncoprotein during the G(0)-to-S phase transition.

c-Fos proto-oncoprotein is rapidly and transiently expressed in cells undergoing the G(0)-to-S phase transition in response to stimulation for growth by serum. Under these conditions, the rapid decay of the protein occurring after induction is accounted for by efficient recognition and degradation by the proteasome. PEST motifs are sequences rich in Pro, Glu, Asp, Ser and Thr which have been proposed to constitute protein instability determinants. c-Fos contains three such motifs, one of which comprises the C-terminal 20 amino acids and has already been proposed to be the major determinant of c-Fos instability. Using site-directed mutagenesis and an expression system reproducing c-fos gene transient expression in transfected cells, we have analysed the turnover of c-Fos mutants deleted of the various PEST sequences in synchronized mouse embryo fibroblasts. Our data showed no role for the two internal PEST motifs in c-Fos instability. However, deletion of the C-terminal PEST region led to only a twofold stabilization of the protein. Taken together, these data indicate that c-Fos instability during the G0-to-S phase transition is governed by a major non-PEST destabilizer and a C-terminal degradation-accelerating element. Further dissection of c-Fos C-terminal region showed that the degradation-accelerating effect is not contributed by the whole PEST sequence but by a short PTL tripeptide which cannot be considered as a PEST motif and which can act in the absence of any PEST environment. Interestingly, the PTL motif is conserved in other members of the fos multigene family. Nevertheless, its contribution to protein instability is restricted to c-Fos suggesting that the mechanisms whereby the various Fos proteins are broken down are, at least partially, different. MAP kinases-mediated phosphorylation of two serines close to PTL, which are both phosphorylated all over the G(0)-to-S phase transition, have been proposed by others to stabilize c-Fos protein significantly. We, however, showed that the PTL motif does not exert its effect by counteracting a stabilizing effect of these phosphorylations under our experimental conditions.

3T3 Cells↗
PubMed2001

Cellular and viral Fos proteins are degraded by different proteolytic systems.

c-Fos proto-oncoprotein is a short-lived transcription factor degraded by the proteasome in vivo. Its mutated forms expressed by the mouse osteosarcomatogenic retroviruses, FBJ-MSV and FBR-MSV, are stabilized two- and threefold, respectively. To elucidate the mechanisms underlying v-Fos(FBJ) and v-Fos(FBR) protein stabilization, we conducted a genetic analysis in which the half-lives and the sensitivities to various cell-permeable protease inhibitors of a variety of cellular and viral protein mutants were measured. Our data showed that the decreased degradation of v-Fos(FBJ) and v-Fos(FBR) is not simply explained by the deletion of a c-Fos destabilizing C-terminal domain. Rather, it involves a complex balance between opposing destabilizing and stabilizing mutations which are distinct and which include virally-introduced peptide motifs in both cases. The mutations in viral Fos proteins conferred both total insensitivity to proteasomal degradation and sensitivity to another proteolytic system not naturally operating on c-Fos, explaining the limited stabilization of the two proteins. This observation is consistent with the idea that FBR-MSV and FBJ-MSV expression machineries have evolved to ensure controlled protein levels. Importantly, our data illustrate that the degradation of unstable proteins does not necessarily involve the proteasome and provide support to the notion that highly related proteins can be broken down by different proteolytic systems in living cells.

Amino Acid Sequence↗
PubMedDate not supplied

Degradation of cellular and viral Fos proteins.

c-Fos proto-oncoprotein is a short-lived transcription factor with oncogenic potential. We have shown that it is massively degraded by the proteasome in vivo under various experimental conditions. Other proteolytic systems including lysosomes and calpains, might, however, also marginally operate on it. Although there is evidence that c-Fos can be ubiquitinylated in vitro, the unambiguous demonstration that ubiquitinylation is necessary for its addressing to the proteasome in vivo is still lacking. c-Jun, one of the main dimerization partners of c-Fos within the AP-1 transcription complex, is also an unstable protein. Its degradation is clearly proteasome- and ubiquitin-dependent in vivo. Interestingly, several lines of evidence indicate that the addressing of c-Fos and c-Jun to the proteasome is, at least in part, governed by different mechanisms. c-Fos has been transduced by two murine osteosarcomatogenic retroviruses under mutated forms which are more stable and more oncogenic. The stabilization is not simply accounted for by simple deletion of c-Fos main destabilizer but, rather, by a complex balance between opposing destabilizing and stabilizing mutations. Though mutations in viral Fos proteins confer full resistance to proteasomal degradation, stabilization is limited because mutations also entail sensitivity to an unidentified proteolytic system. This observation is consistent with the idea that Fos-expressing viruses have evolved to ensure control protein levels to avoid high protein accumulation-linked apoptosis. In conclusion, the unveiling of the complex mechanism network responsible for the degradation of AP-1 family members is still at its beginning and a number of issues regarding the regulation of this process and the addressing to the proteasome are still unresolved.

Animals↗