Loss of Dictyostelium HSPC300 causes a scar-like phenotype and loss of SCAR protein

<p>Abstract</p> <p>Background</p> <p>SCAR/WAVE proteins couple signalling to actin polymerization, and are thus fundamental to the formation of pseudopods and lamellipods. They are controlled as part of a five-membered complex that includes the tiny HSPC300 protein. It is not known why SCAR/WAVE is found in such a large assembly, but in <it>Dictyostelium </it>the four larger subunits have different, clearly delineated functions.</p> <p>Results</p> <p>We have generated <it>Dictyostelium </it>mutants in which the HSPC300 gene is disrupted. As has been seen in other regulatory complex mutants, SCAR is lost in these cells, apparently by a post-translational mechanism, though PIR121 levels do not change. HSPC300 knockouts resemble <it>scar </it>mutants in slow migration, roundness, and lack of large pseudopods. However <it>hspc300</it>-colonies on bacteria are larger and more similar to wild type, suggesting that some SCAR function can survive without HSPC300. We find no evidence for functions of HSPC300 outside the SCAR complex.</p> <p>Conclusion</p> <p>HSPC300 is essential for most SCAR complex functions. The phenotype of HSPC300 knockouts is most similar to mutants in <it>scar</it>, not the other members of the SCAR complex, suggesting that HSPC300 acts most directly on SCAR itself.</p

Clustering of glycoprotein VI (GPVI) dimers upon adhesion to collagen as a mechanism to regulate GPVI signaling in platelets.

Essentials - Dimeric high-affinity collagen receptor glycoprotein VI (GPVI) is present on resting platelets. - Spatio-temporal organization of platelet GPVI-dimers was evaluated using advanced microscopy. - Upon platelet adhesion to collagenous substrates, GPVI-dimers coalesce to form clusters. - Clustering of GPVI-dimers may increase avidity and facilitate platelet activation SUMMARY: Background Platelet glycoprotein VI (GPVI) binding to subendothelial collagen exposed upon blood vessel injury initiates thrombus formation. Dimeric GPVI has high affinity for collagen, and occurs constitutively on resting platelets. Objective To identify higher-order oligomerization (clustering) of pre-existing GPVI dimers upon interaction with collagen as a mechanism to initiate GPVI-mediated signaling. Methods GPVI was located by use of fluorophore-conjugated GPVI dimer-specific Fab (antigen-binding fragment). The tested substrates include Horm collagen I fibers, soluble collagen III, GPVI-specific collagen peptides, and fibrinogen. GPVI dimer clusters on the platelet surface interacting with these substrates were visualized with complementary imaging techniques: total internal reflection fluorescence microscopy to monitor real-time interactions, and direct stochastic optical reconstruction microscopy (dSTORM), providing relative quantification of GPVI cluster size and density. Confocal microscopy was used to locate GPVI dimer clusters, glycoprotein Ib, integrin α$_{2}$β$_{1}$ , and phosphotyrosine. Results Upon platelet adhesion to all collagenous substrates, GPVI dimers coalesced to form clusters; notably clusters formed along the fibers of Horm collagen. dSTORM revealed that GPVI density within clusters depended on the substrate, collagen III being the most effective. Clusters on fibrinogen-adhered platelets were much smaller and more numerous; whether these are pre-existing oligomers of GPVI dimers or fibrinogen-induced is not clear. Some GPVI dimer clusters colocalized with areas of phosphotyrosine, indicative of signaling activity. Integrin α$_{2}$β$_{1}$ was localized to collagen fibers close to GPVI dimer clusters. GPVI clustering depends on a dynamic actin cytoskeleton. Conclusions Platelet adhesion to collagen induces GPVI dimer clustering. GPVI clustering increases both avidity for collagen and the proximity of GPVI-associated signaling molecules, which may be crucial for the initiation and persistence of signaling.These studies were supported by a Project Grant (PG/10/011/28199, to S. M. Jung, M. Moroi, R. W. Farndale, and S. P. Watson) and a Special Project Grant (SP/13/7/30575, to S. M. Jung) from the British Heart Foundation and a Wellcome Trust Biomedical Resource Grant (09440/Z/10/Z, to R. W. Farndale). S. P. Watson and N. S. Poulter are supported by the British Heart Foundation (CH/03/003). A. Y. Pollitt was funded by Wellcome Trust Grant 088410 (to S. P. Watson)

High-Resolution X-Ray Structure of the Trimeric Scar/WAVE-Complex Precursor Brk1

The Scar/WAVE-complex links upstream Rho-GTPase signaling to the activation of the conserved Arp2/3-complex. Scar/WAVE-induced and Arp2/3-complex-mediated actin nucleation is crucial for actin assembly in protruding lamellipodia to drive cell migration. The heteropentameric Scar/WAVE-complex is composed of Scar/WAVE, Abi, Nap, Pir and a small polypeptide Brk1/HSPC300, and recent work suggested that free Brk1 serves as a homooligomeric precursor in the assembly of this complex. Here we characterized the Brk1 trimer from Dictyostelium by analytical ultracentrifugation and gelfiltration. We show for the first time its dissociation at concentrations in the nanomolar range as well as an exchange of subunits within different DdBrk1 containing complexes. Moreover, we determined the three-dimensional structure of DdBrk1 at 1.5 Å resolution by X-ray crystallography. Three chains of DdBrk1 are associated with each other forming a parallel triple coiled-coil bundle. Notably, this structure is highly similar to the heterotrimeric α-helical bundle of HSPC300/WAVE1/Abi2 within the human Scar/WAVE-complex. This finding, together with the fact that Brk1 is collectively sandwiched by the remaining subunits and also constitutes the main subunit connecting the triple-coil domain of the HSPC300/WAVE1/Abi2/ heterotrimer to Sra1(Pir1), implies a critical function of this subunit in the assembly process of the entire Scar/WAVE-complex

Pleiotropic Roles of a Ribosomal Protein in Dictyostelium discoideum

The cell cycle phase at starvation influences post-starvation differentiation and morphogenesis in Dictyostelium discoideum. We found that when expressed in Saccharomyces cerevisiae, a D. discoideum cDNA that encodes the ribosomal protein S4 (DdS4) rescues mutations in the cell cycle genes cdc24, cdc42 and bem1. The products of these genes affect morphogenesis in yeast via a coordinated moulding of the cytoskeleton during bud site selection. D. discoideum cells that over- or under-expressed DdS4 did not show detectable changes in protein synthesis but displayed similar developmental aberrations whose intensity was graded with the extent of over- or under-expression. This suggested that DdS4 might influence morphogenesis via a stoichiometric effect – specifically, by taking part in a multimeric complex similar to the one involving Cdc24p, Cdc42p and Bem1p in yeast. In support of the hypothesis, the S. cerevisiae proteins Cdc24p, Cdc42p and Bem1p as well as their D. discoideum cognates could be co-precipitated with antibodies to DdS4. Computational analysis and mutational studies explained these findings: a C-terminal domain of DdS4 is the functional equivalent of an SH3 domain in the yeast scaffold protein Bem1p that is central to constructing the bud site selection complex. Thus in addition to being part of the ribosome, DdS4 has a second function, also as part of a multi-protein complex. We speculate that the existence of the second role can act as a safeguard against perturbations to ribosome function caused by spontaneous variations in DdS4 levels

The study of platelet receptors using artificial lipid bilayers

Artificial lipid bilayers are powerful tools that can be used to model the interactions between platelets and membrane-bound ligands. To mimic the interaction of platelets with membrane-bound ligands, biotinylated lipids can be used to couple monobiotinylated recombinant ligands to the upper leaflet of an artificial lipid bilayer using streptavidin to bridge the two. Artificial lipid bilayers are generated by preparing liposomes, treating glass coverslips to make them hydrophilic and by assembling the bilayer in a specialized flow chamber. Finally platelets can be added to the flow chamber and the localization of fluorescently labeled molecules followed using microscopy

The study of platelet receptors using artificial lipid bilayers

Artificial lipid bilayers are powerful tools that can be used to model the interactions between platelets and membrane-bound ligands. To mimic the interaction of platelets with membrane-bound ligands, biotinylated lipids can be used to couple monobiotinylated recombinant ligands to the upper leaflet of an artificial lipid bilayer using streptavidin to bridge the two. Artificial lipid bilayers are generated by preparing liposomes, treating glass coverslips to make them hydrophilic and by assembling the bilayer in a specialized flow chamber. Finally platelets can be added to the flow chamber and the localization of fluorescently labeled molecules followed using microscopy

OFICINA DE CANTO POPULAR

We have recently shown that the C-type lectin-like receptor, CLEC-2, is expressed on platelets and that it mediates powerful platelet aggregation by the snake venom toxin rhodocytin. In addition, we have provided indirect evidence for an endogenous ligand for CLEC-2 in renal cells expressing HIV-1. This putative ligand facilitates transmission of HIV through its incorporation into the viral envelope and binding to CLEC-2 on platelets. The aim of the present study was to identify the ligand on these cells which binds to CLEC-2 on platelets. Recombinant CLEC-2 exhibits specific binding to HEK-293T (human embryonic kidney) cells in which the HIV can be grown. Furthermore, HEK-293T cells activate both platelets and CLEC-2-transfected DT-40 B-cells. The transmembrane protein podoplanin was identified on HEK-293T cells and was demonstrated to mediate both binding of HEK-293T cells to CLEC-2 and HEK-293T cell activation of CLEC-2-transfected DT-40 B-cells. Podoplanin is expressed on renal cells (podocytes). Furthermore, a direct interaction between CLEC-2 and podoplanin was confirmed using surface plasmon resonance and was shown to be independent of glycosylation of CLEC-2. The interaction has an affinity of 24.5+/-3.7 microM. The present study identifies podoplanin as a ligand for CLEC-2 on renal cells