Sorting of lipoproteins to the outer membrane in E. coli

Escherichia coli lipoproteins are anchored to the periplasmic surface of the inner or outer membrane depending on the sorting signal. An ATP-binding cassette (ABC) transporter, LolCDE, releases outer membrane-specific lipoproteins from the inner membrane, causing the formation of a complex between the released lipoproteins and the periplasmic molecular chaperone LolA. When this complex interacts with outer membrane receptor LolB, the lipoproteins are transferred from LolA to LolB and then localized to the outer membrane. The structures of LolA and LolB are remarkably similar to each other. Both have a hydrophobic cavity consisting of an unclosed beta-barrel and an alpha-helical lid. Structural differences between the two proteins reveal the molecular mechanisms underlying the energy-independent transfer of lipoproteins from LolA to LolB. Strong inner membrane retention of lipoproteins occurs with Asp at position 2 and a few limited residues at position 3. The inner membrane retention signal functions as a Lol avoidance signal and inhibits the recognition of lipoproteins by LolCDE, thereby causing their retention in the inner membrane. The positive charge of phosphatidylethanolamine and the negative charge of Asp at position 2 are essential for Lol avoidance. The Lol avoidance signal is speculated to cause the formation of a tight lipoprotein-phosphatidylethanolamine complex that has five acyl chains and therefore cannot be recognized by LolCDE.     

Notch governing mature T cell differentiation

The differentiation of naive T cells to effector/memory T cells is regulated by a variety of factors. The recent advance of the contribution of Notch signaling in this differentiation step has provided a new path to better understand the acquisition or persistence of effector function of mature T cells. In this review, we summarize emerging and, in some points, conflicting evidence for Notch signaling on mature T cell activation and differentiation.     

Fission Yeast Germinal Center (GC) Kinase Ppk11 Interacts with Pmo25 and Plays an Auxiliary Role in Concert with the Morphogenesis Orb6 Network (MOR) in Cell Morphogenesis

How cell morphology and the cell cycle are coordinately regulated is a fundamental subject in cell biology. In fission yeast, 2 germinal center kinases (GCKs), Sid1 and Nak1, play an essential role in septation/cytokinesis and cell separation/cell polarity control, respectively, as components of the septation initiation network (SIN) and the morphogenesis Orb6 network (MOR). Here we show that a third GCK, Ppk11, is also required for efficient cell separation particularly, at a high temperature. Although Ppk11 is not essential for cell division, this kinase plays an auxiliary role in concert with MOR in cell morphogenesis. Ppk11 physically interacts with the MOR component Pmo25 and is localized to the septum, by which Ppk11 is crucial for Pmo25 targeting/accumulation to the septum. The conserved C-terminal WDF motif of Ppk11 is essential for both septum accumulation of Pmo25 and efficient cell separation. In contrast its kinase activity is required only for cell separation. Thus, both interaction of Ppk11 with Pmo25 and Ppk11 kinase activity are critical for efficient cell separation.     

Mitochondrial respiration defects modulate differentiation but not proliferation of hematopoietic stem and progenitor cells

Mitochondrial energy production is involved in various cellular processes. Here we show that ATP content is significantly increased in lineage-restricted progenitor cells compared with hematopoietic stem and progenitor cells (HSPCs) or more differentiated cells. Transplantation analysis using a mouse model of mitochondrial disease revealed that mitochondrial respiration defects resulted in a significant decrease in the total number and repopulating activity of bone marrow cells, although the number of HSPCs increased. The proliferative activity of HSPCs and lineage-restricted progenitor cells was not impaired by reduction of ATP content and there seems to be no associated increase in reactive oxygen species levels and apoptosis. Our findings indicate that mitochondrial respiration defects modulate HSPC commitment/differentiation into lineage-restricted progenitor cells.     

Occurrence of the Fimbria Gene hifA in clinical isolates of nonencapsulated Haemophilus influenzae

The adherence of Haemophilus influenzae to epithelial cells plays a crucial role in infections. However, little is known about the occurrence of fimbriae. In this study, we examined the distribution of the fimbria gene (hifA) by PCR among 167 H. influenzae strains isolated from patients with respiratory infections. Almost all (163; 98%) of the isolates were nonencapsulated strains. The carriage rate of hifA by the nonencapsulated strains was 18.4%. Electron microscopy showed that fimbriae were abundantly present on the cell surface of hifA-positive strains tested. Only four (2.4%) isolates were encapsulated, all of which were type b and did not possess hifA. The present work suggests that fimbriae may play a considerable role as adhesins in nonencapsulated H. influenzae strains.     

Regulation of nitrate reductase by non-modifiable derivatives of PII in the cells of Synechococcus elongatus strain PCC 7942

In Synechococcus elongatus, the PII protein inhibits both transport and reduction of nitrate when ammonium is present in the medium. Using a transporter mutant having ammonium-resistant nitrate transport activity as the genetic background, we analyzed specific effects of PII on in vivo nitrate reductase activity by measuring uptake of nitrate from the medium. The results showed that the regulation of nitrate reductase does not require changes in the electric charge or size of the side chain at the phosphorylation site of PII. Phosphorylation of PII is thus unlikely to play a role in the regulation of nitrate reductase.     

Plant lanosterol synthase: divergence of the sterol and triterpene biosynthetic pathways in eukaryotes

Sterols, essential eukaryotic constituents, are biosynthesized through either cyclic triterpenes, lanosterol (fungi and animals) or cycloartenol (plants). The cDNA for OSC7 of Lotus japonicus was shown to encode lanosterol synthase (LAS) by the complementation of a LAS-deficient mutant yeast and structural identification of the accumulated lanosterol. A double site-directed mutant of OSC7, in which amino acid residues crucial for the reaction specificity were changed to the cycloartenol synthase (CAS) type, produced parkeol and cycloartenol. The multiple amino acid sequence alignment of a conserved region suggests that the LAS of different eukaryotic lineages emerged from the ancestral CAS by convergent evolution.