Inter-Species Complementation of the Translocon Beta Subunit Requires Only Its Transmembrane Domain

In eukaryotes, proteins enter the secretory pathway through the translocon pore of the endoplasmic reticulum. This protein translocation channel is composed of three major subunits, called Sec61α, β and γ in mammals. Unlike the other subunits, the β subunit is dispensable for translocation and cell viability in all organisms studied. Intriguingly, the knockout of the Sec61β encoding genes results in different phenotypes in different species. Nevertheless, the β subunit shows a high level of sequence homology across species, suggesting the conservation of a biological function that remains ill-defined. To address its cellular roles, we characterized the homolog of Sec61β in the fission yeast Schizosaccharomyces pombe (Sbh1p). Here, we show that the knockout of sbh1+ results in severe cold sensitivity, increased sensitivity to cell-wall stress, and reduced protein secretion at 23°C. Sec61β homologs from Saccharomyces cerevisiae and human complement the knockout of sbh1+ in S. pombe. As in S. cerevisiae, the transmembrane domain (TMD) of S. pombe Sec61β is sufficient to complement the phenotypes resulting from the knockout of the entire encoding gene. Remarkably, the TMD of Sec61β from S. cerevisiae and human also complement the gene knockouts in both yeasts. Together, these observations indicate that the TMD of Sec61β exerts a cellular function that is conserved across species

Accommodating a Non-Conservative Internal Mutation by WaterMediated Hydrogen-Bonding Between β-Sheet Strands: A Comparison of Human and Rat Type B (Mitochondrial) Cytochrome b5

Mammalian type B (mitochondrial) cytochromes b5 exhibit greater amino acid sequence diversity than their type A (microsomal) counterparts, as exemplified by the type B proteins from human (hCYB5B) and rat (rCYB5B). The comparison of X-ray crystal structures of hCYB5B and rCYB5B reported herein reveals a striking difference in packing involving the five-stranded β-sheet, attributable to fully buried residue 21 in strand β4. The greater bulk of Leu21 in hCYB5B in comparison to Thr21 in rCYB5B results in a substantial displacement of the first two residues in β5, and consequent loss of two of the three hydrogen bonds between β5 and β4. Hydrogen-bonding between the residues is instead mediated by two well-ordered, fully buried water molecules. In a 10 ns molecular dynamics simulation, one of the buried water molecules in the hCYB5B structure exchanged readily with solvent via intermediates having three water molecules sandwiched between β4 and β5. When the buried water molecules were removed prior to a second 10 ns simulation, β4 and β5 formed persistent hydrogen bonds identical to those in rCYB5B, but the Leu21 side chain was forced to adopt a rarely observed conformation. Despite the apparently greater ease of water access to the interior of hCYB5B than of rCYB5B suggested by these observations, the two proteins exhibit virtually identical stability, dynamic and redox properties. The results provide new insight into the factors stabilizing the cytochrome b5 fold

Inter-Species Complementation of the Translocon Beta Subunit Requires Only Its Transmembrane Domain

In eukaryotes, proteins enter the secretory pathway through the translocon pore of the endoplasmic reticulum. This protein translocation channel is composed of three major subunits, called Sec61α, β and γ in mammals. Unlike the other subunits, the β subunit is dispensable for translocation and cell viability in all organisms studied. Intriguingly, the knockout of the Sec61β encoding genes results in different phenotypes in different species. Nevertheless, the β subunit shows a high level of sequence homology across species, suggesting the conservation of a biological function that remains ill-defined. To address its cellular roles, we characterized the homolog of Sec61β in the fission yeast Schizosaccharomyces pombe (Sbh1p). Here, we show that the knockout of sbh1+ results in severe cold sensitivity, increased sensitivity to cell-wall stress, and reduced protein secretion at 23°C. Sec61β homologs from Saccharomyces cerevisiae and human complement the knockout of sbh1+ in S. pombe. As in S. cerevisiae, the transmembrane domain (TMD) of S. pombe Sec61β is sufficient to complement the phenotypes resulting from the knockout of the entire encoding gene. Remarkably, the TMD of Sec61β from S. cerevisiae and human also complement the gene knockouts in both yeasts. Together, these observations indicate that the TMD of Sec61β exerts a cellular function that is conserved across species

Amikacin, ceftazidime, and flucloxacillin against suspended and adherent pseudomonas aeruginosa and staphylococcus epidermidis in an In vitro model of infection

Bacterial inocula were exposed as suspended cultures or as adherent biofilms on glass beads in a novel in vitro model of infection to oscillating drug concentrations mimicking human serum kinetics during clinical treatment. Amikacin was given once or thrice daily alone or in combination with ceftazidime or flucloxacillin against Pseudomonas aeruginosa or Staphylococcus epidermidis. Killing of adherent bacteria was significantly reduced during single-drug treatment compared with suspended bacteria (P < .001), and β-lactams were more active than amikacin against both suspended and adherent bacteria (P < .01), Amikacin-β-lactam combinations killed the inocula more rapidly and were consistently bactericidal against both suspended and adherent pathogens (P < .05). Once-daily dosing of amikacin produced greater initial killing than thrice daily dosing (P < .05), but both regimens were similarly effective after 48 h. The differences in antibiotic activity against suspended and adherent bacteria may relate to clinical failures in the treatment of foreign-body infections by bacteria sensitive to the administered antibiotics, as determined by standard susceptibility test

Genome sequence of Ostreococcus tauri virus OtV-2 enlightens the role of picoeukaryote niche separation in the ocean

Ostreococcus tauri, a unicellular marine green alga, is the smallest known free-living eukaryote and is ubiquitous in the surface oceans. The ecological success of this organism has been attributed to distinct low- and high-light adapted ecotypes existing in different niches at a range of depths in the ocean. Viruses have already been characterised that infect the high-light adapted strains. Ostreococcus tauri virus isolate OtV-2 is a large double stranded DNA algal virus that infects a low-light adapted strain of O. tauri and was assigned to the algal virus family Phycodnaviridae, genus Prasinovirus. Our working hypothesis for this study was that different viruses infecting high-light vs. low-light adapted O. tauri strains would provide clues to propagation strategies that would give them selective advantages within their particular light niche. Sequence analysis of the 184,409 base pair linear OtV-2 genome revealed a range of core functional genes exclusive to this low-light genotype and included a variety of unexpected genes, such as those encoding a RNA polymerase sigma factor, at least four DNA methyltransferases, a cytochrome b5 and a high affinity phosphate transporter. It is clear that OtV-2 has acquired a range of potentially functional genes from its host, other eukaryotes and even bacteria over evolutionary time. Such piecemeal accretion of genes is a trademark of large doublestranded DNA viruses that has allowed them to adapt their propagation strategies to keep up with host niche separation in the sunlit layers of the oceanic environment