Aminophospholipid translocase in the plasma membrane of Friend erythroleukemic cells can induce an asymmetric topology for phosphatidylserine but not for phosphatidylethanolamine

The ATP-dependent translocation of phospholipids in the plasma membrane of intact Friend erythroleukemic cells (FELCs) was studied in comparison with that in the membrane of mature murine erythrocytes. This was done by following the fate of radiolabeled phospholipid molecules, previously inserted into the outer monolayer of the plasma membranes by using a non-specific lipid transfer protein. The transbilayer equilibration of these probe molecules was monitored by treating the cells--under essentially non-lytic conditions--with phospholipases A2 of different origin. Rapid reorientations of the newly introduced aminophospholipids in favour of the inner membrane leaflet were observed in fresh mouse erythrocytes; the inward translocation of phosphatidylcholine (PC) in this membrane proceeded relatively slow. In FELCs, on the other hand, all three glycerophospholipids equilibrated over both halves of the plasma membrane very rapidly, i.e. within 1 h; nevertheless, an asymmetric distribution in favour of the inner monolayer was only observed for phosphatidylserine (PS). Lowering the ATP-level in the FELCs caused a reduction in the rate of inward translocation of both aminophospholipids, but not of that of PC, indicating that this translocation of PS and phosphatidylethanolamine (PE) is clearly ATP-dependent. Hence, the situation in the plasma membrane of the FELC is rather unique in a sense that, though an ATP-dependent translocase is present and active both for PS and PE, its activity results in an asymmetric distribution of PS, but not of PE. This remarkable situation might be the consequence of the fact that, in contrast to the mature red cell, this precursor cell still lacks a complete membrane skeletal network.     

Expression of the Opc protein correlates with invasion of epithelial and endothelial cells by Neisseria meningitidis

Whereas capsulate strains of Neisseria meningitidis are dependent on pili for adhesion to human endothelial and epithelial cells, strains which lacked assembled pili and were partially capsule-deficient adhered to and invaded human endothelial and epithelial cells if they expressed the Opc protein. Bacteria expressing low or undetectable levels of Opc protein failed to adhere to or invade eukaryotic cells. In addition, the presence of OpaAC751 protein on the surface of bacteria did not increase bacterial interactions with host cells. Association of Opc-expressing bacteria was inhibited by antibodies against Opc. Invasion was dependent on the host-cell cytoskeletal activity and was inhibited by cytochalasin D. In some cells, infected at the apical surface, bacteria emerging from basal surface were detected by electron microscopy. Opc is found in diverse meningococci and may represent a common virulence factor which facilitates adherence and invasion by these bacteria.     

A rapid method purifies a glycoprotein of Mr 145,000 as the LDL receptor of Trypanosoma brucei brucei

The trypanosome LDL receptor has been isolated from bloodstream form and cultured insect-stage trypanosomes as a protein of Mr 145,000, using a rapid purification procedure in the presence of a cocktail of protease inhibitors, whereas previously a polypeptide of Mr 86,000 was purified as the LDL receptor. Both the 145,000 and the 86,000 polypeptides are glycosylated and recognized by a monospecific antibody raised against the 86,000 species. This antibody inhibits LDL binding to the intact trypanosomes, to the isolated 145,000 receptor and to the 86,000 species. Hence, the previously isolated 86,000 polypeptide is a degradation product probably representing the cleaved-off ectodomain of the trypanosome LDL receptor.     

Mixed Layer Depth Seasonality within the Coral Sea Based on Argo Data

The worldwide deployment of Argo floats has enabled much more detailed studies of global and regional seas over the last decade. Here, the seasonal variability of the mixed layer depth (MLD) within the Coral Sea was examined with CTD profiles from Argo floats. Multiple threshold values for both temperature and density have been employed to determine the most suitable threshold values for the Coral Sea. A threshold value of 0.04 kg/m³ for density and 0.2°C for temperature appear the most fitting for this region. Although MLD and isothermal layer depth (ILD) coincide quite well in most cases, the relatively common presence of temporary, non-seasonal barrier layers induces an ILD that is significantly deeper than the MLD. Consequently, an MLD estimation based on density is more appropriate. A distinct seasonality in the MLD is evident throughout the Coral Sea, but is generally more pronounced in higher southern latitudes (20–30°S). Salinity inversions are rare and mainly occur in the south-eastern Coral Sea, while barrier layers are more commonly associated with the north-eastern Coral Sea, a region characterised by high rainfall. The significance of regional currents is evident in the north-western Coral Sea, where temperature and ocean heat content is relatively low due to a northward moving boundary current. Shallow bathymetry, in turn, is linked to the absence of Argo data on the continental shelf and in the central Coral Sea.     

A single Na+-Pi cotransporter in Toxoplasma plays key roles in phosphate import and control of parasite osmoregulation

Inorganic ions such as phosphate, are essential nutrients required for a broad spectrum of cellular functions and regulation. During infection, pathogens must obtain inorganic phosphate (P_i) from the host. Despite the essentiality of phosphate for all forms of life, how the intracellular parasite Toxoplasma gondii acquires P_i from the host cell is still unknown. In this study, we demonstrated that Toxoplasma actively internalizes exogenous P_i by exploiting a gradient of Na⁺ ions to drive P_i uptake across the plasma membrane. The Na⁺-dependent phosphate transport mechanism is electrogenic and functionally coupled to a cipargarmin sensitive Na⁺-H⁺-ATPase. Toxoplasma expresses one transmembrane P_i transporter harboring PHO4 binding domains that typify the PiT Family. This transporter named TgPiT, localizes to the plasma membrane, the inward buds of the endosomal organelles termed VAC, and many cytoplasmic vesicles. Upon P_i limitation in the medium, TgPiT is more abundant at the plasma membrane. We genetically ablated the PiT gene, and ΔTgPiT parasites are impaired in importing P_i and synthesizing polyphosphates. Interestingly, ΔTgPiT parasites accumulate 4-times more acidocalcisomes, storage organelles for phosphate molecules, as compared to parental parasites. In addition, these mutants have a reduced cell volume, enlarged VAC organelles, defects in calcium storage and a slightly alkaline pH. Overall, these mutants exhibit severe growth defects and have reduced acute virulence in mice. In survival mode, ΔTgPiT parasites upregulate several genes, including those encoding enzymes that cleave or transfer phosphate groups from phosphometabolites, transporters and ions exchangers localized to VAC or acidocalcisomes. Taken together, these findings point to a critical role of TgPiT for P_i supply for Toxoplasma and also for protection against osmotic stresses.