Microtubule-independent secretion requires functional maturation of Golgi elements.

The Golgi complex is responsible for processing and sorting of secretory cargos. Microtubules are known to accelerate the transport of proteins from the endoplasmic reticulum (ER) to the Golgi complex and from the Golgi to the plasma membrane. However, whether post-Golgi transport strictly requires microtubules is still unclear. Using the retention using selective hooks (RUSH) system to synchronize the trafficking of cargos, we show that anterograde transport of tumor necrosis factor (TNF) is strongly reduced without microtubules. We show that two populations of Golgi elements co-exist in these cells. A centrally located and giantin-positive Golgi complex that sustains trafficking, and newly formed peripheral Golgi mini-stacks that accumulate cargos in cells without microtubules. Using a genome-edited GFP-giantin cell line, we observe that the trafficking-competent Golgi population corresponds to the pre-existing population that was present before removal of microtubules. All Golgi elements support trafficking after long-term depletion of microtubules and after relocation of Golgi proteins to the ER after treatment with Brefeldin A. Our results demonstrate that functional maturation of Golgi elements is needed to ensure post-Golgi trafficking, and that microtubule-driven post-Golgi transport is not strictly required.

Synchronization of secretory protein traffic in populations of cells.

To dissect secretory traffic, we developed the retention using selective hooks (RUSH) system. RUSH is a two-state assay based on the reversible interaction of a hook protein fused to core streptavidin and stably anchored in the donor compartment with a reporter protein of interest fused to streptavidin-binding peptide (SBP). Biotin addition causes a synchronous release of the reporter from the hook. Using the RUSH system, we analyzed different transport characteristics of various Golgi and plasma membrane reporters at physiological temperature in living cells. Using dual-color simultaneous live-cell imaging of two cargos, we observed intra- and post-Golgi segregation of cargo traffic, consistent with observation in other systems. We show preliminarily that the RUSH system is usable for automated screening. The system should help increase the understanding of the mechanisms of trafficking and enable screens for molecules that perturb pathological protein transport.

Lysosomal membrane proteins LAMP1 and LIMP2 are segregated in the Golgi apparatus independently of their clathrin adaptor binding motif.

To reach the lysosome, lysosomal membrane proteins (LMPs) are translocated in the endoplasmic reticulum after synthesis and then transported to the Golgi apparatus. The existence of a direct transport from the Golgi apparatus to the endosomes but also of an indirect route through the plasma membrane has been described. Clathrin adaptor binding motifs contained in the cytosolic tail of LMPs have been described as key players in their intracellular trafficking. Here we used the RUSH assay to synchronize the biosynthetic transport of multiple LMPs. After exiting the Golgi apparatus, RUSH-synchronized LAMP1 was addressed to the cell surface both after overexpression or at endogenous level. Its YXXΦ motif was not involved in the transport from the Golgi apparatus to the plasma membrane but in its endocytosis. LAMP1 and LIMP2 were sorted from each other after reaching the Golgi apparatus. LIMP2 was incorporated in punctate structures for export from the Golgi apparatus from which LAMP1 is excluded. LIMP2-containing post-Golgi transport intermediates did not rely neither on its adaptor binding signal nor on its C-terminal cytoplasmic domain.

Stiffness tomography of eukaryotic intracellular compartments by atomic force microscopy.

Precise localization and biophysical characterization of cellular structures is a key to the understanding of biological processes happening both inside the cell and at the cell surface. Atomic force microscopy is a powerful tool to study the cell surface - topography, elasticity, viscosity, interactions - and also the viscoelastic behavior of the underlying cytoplasm, cytoskeleton or the nucleus. Here, we demonstrate the ability of atomic force microscopy to also map and characterize organelles and microorganisms inside cells, at the nanoscale, by combining stiffness tomography with super-resolution fluorescence and electron microscopy. By using this correlative approach, we could both identify and characterize intracellular compartments. The validation of this approach was performed by monitoring the stiffening effect according to the metabolic status of the mitochondria in living cells in real-time.

Recruitment of LC3 to damaged Golgi apparatus.

LC3 is a protein that can associate with autophagosomes, autolysosomes, and phagosomes. Here, we show that LC3 can also redistribute toward the damaged Golgi apparatus where it clusters with SQSTM1/p62 and lysosomes. This organelle-specific relocation, which did not involve the generation of double-membraned autophagosomes, could be observed after Golgi damage was induced by various strategies, namely (i) laser-induced localized cellular damage, (ii) local expression of peroxidase and exposure to peroxide and diaminobenzidine, (iii) treatment with the Golgi-tropic photosensitizer redaporfin and light, (iv) or exposure to the Golgi-tropic anticancer peptidomimetic LTX-401. Mechanistic exploration led to the conclusion that both reactive oxygen species-dependent and -independent Golgi damage induces a similar phenotype that depended on ATG5 yet did not depend on phosphatidylinositol-3-kinase catalytic subunit type 3 and Beclin-1. Interestingly, knockout of ATG5 sensitized cells to Golgi damage-induced cell death, suggesting that the pathway culminating in the relocation of LC3 to the damaged Golgi may have a cytoprotective function.

Activation of Epstein-Barr virus/C3d receptor (gp140, CR2, CD21) on human B lymphoma cell surface triggers Cbl tyrosine phosphorylation, its association with p85 subunit, Crk-L and Syk and its dissociation with Vav.

It is well established that CD21 activation on human B cell surface triggers B cell proliferation. We previously demonstrated that CD21 activation also triggers tyrosine phosphorylation of two components, p95 and p120, both interacting with SH2 domains of the p85 subunit of PI 3-kinase. We successively identified p95 as the nucleolin and the first signal transduction pathway specifically triggered by CD21 activation, i.e.: pp60Src activation, tyrosine phosphorylation of p95 nucleolin, its interaction with SH2 domains of p85 subunit and PI 3-kinase activation, followed by AKT-GSK-3 activations. We herein identified the p120 component as the protooncoprotein Cbl and the first steps associated to its activation. First, CD21 activation triggered Cbl tyrosine phosphorylation, which required c-Src kinase but not PI 3-kinase or Syk kinase activities. Involvement of Src kinase in this step was supported by inhibition of Cbl phosphorylation and its interactions with other components when cells were either preincubated with specific Src inhibitor or transfected with dominant-negative c-Src form. Second, once tyrosine phosphorylated, Cbl interacts with SH2 domains of p85 subunit, SH2 domains of Crk-L and with tyrosine phosphorylated Syk kinase. The third and unexpected feature was to found that, at the contrary of BCR or of CD19 (herein also analyzed for the first time), CD21 activation triggers dissociation of Cbl-Vav complex. Thus, these results provide the first molecular basis of a new signal transduction pathway specifically triggered by CD21 activation.

RB18A enhances expression of mutant p53 protein in human cells.

RB18A (TRAP220/DRIP205) is a cofactor of transcription. We herein demonstrated that RB18A downregulated p53 and upregulated MDM2 promoters. These RB18A regulations, not modified by p53wt expression, were inhibited by mutant p53 (p53mut) expression, which directly interacts with RB18A D5 domain. In addition, RB18A via its D4 domain, also interacts directly and specifically with MDM2 protein inhibiting p53mut degradation. Altogether, these mechanisms contribute to maintain a high level of p53mut expression in tumor proliferating cells. Therefore, RB18A plays a central role to control p53wt and p53mut protein content and functions in cells through a loop of regulation, which involves MDM2.

Infection of human B lymphoma cells by Mycoplasma fermentans induces interaction of its elongation factor with the intracytoplasmic domain of Epstein-Barr virus receptor (gp140, EBV/C3dR, CR2, CD21).

Human cell lines are often infected by mycoplama strains. We have demonstrated that when infected by Mycoplasma fermentans, human B lymphoma cell proliferation increased strongly. These infected B cells expressed a p45 kDa protein which interacted with the intracellular domain of CD21, the EBV/C3d receptor. p45 analysis demonstrated that this is a new gene which encodes an elongation factor originating from Mycoplasma fermentans. p45 interaction with CD21 was specific, there being no interaction with CD19. This is the first demonstration that Mycoplasma fermentans, in infecting human B cells, generates a p45 Mycoplasma component that interacts with CD21, which is involved in B cell proliferation.

ESCRT machinery is required for plasma membrane repair.

Plasma membrane damage can be triggered by numerous phenomena, and efficient repair is essential for cell survival. Endocytosis, membrane patching, or extracellular budding can be used for plasma membrane repair. We found that endosomal sorting complex required for transport (ESCRT), involved previously in membrane budding and fission, plays a critical role in plasma membrane repair. ESCRT proteins were recruited within seconds to plasma membrane wounds. Quantitative analysis of wound closure kinetics coupled to mathematical modeling suggested that ESCRTs are involved in the repair of small wounds. Real-time imaging and correlative scanning electron microscopy (SEM) identified extracellular buds and shedding at the site of ESCRT recruitment. Thus, the repair of certain wounds is ensured by ESCRT-mediated extracellular shedding of wounded portions.

Fully in vitro selection of recombinant antibodies.

Antibodies are essential for the identification and characterization of proteins. In the current postgenomic era the need for highly specific antibodies has further increased not only for research applications but also because they represent one of the most promising therapeutic options, especially in the field of cancer treatment. One appealing approach for rapid and inexpensive antibody generation is the use of phage display. This technique allows for a fast and animal-free selection of highly functional alternatives to classical antibodies. However, one strong limitation of this recombinant approach has been the difficulty in producing and purifying antigens. These steps have to be adjusted for each new target, are time consuming and sometimes present an insurmountable obstacle. Here we report the development of new antibody selection approach where antigens are produced through in vitro translation and are used directly and without the need for purification. With this approach we were able to rapidly select recombinant antibodies directed against GFP and the mammalian protein tsg101, respectively. We believe that our method greatly facilitates antigen preparation and thus may broaden the use of the recombinant approach for antibody generation, especially since the technique could in the future be adapted to a high-throughput technology, thus further accelerating antibody selection.