Isolation and structural studies on a galactofuranosyl-containing glycopeptide fromAscobolus furfuraceus

A galactofuranosyl-containing glycopeptide has been isolated from mycelium ofAscobolus furfuraceus by extraction with water. The glycoconjugate was purified by DEAE-cellulose chromatography followed by gel filtration. A molecular weight of about 20 000 was determined by the latter method using standard dextrans. Neutral sugars accounted for 94.5% of the glycopeptide and were characterized as mannose, galactose, and glucose. Glucosamine was estimated colorimetrically (1.8%). The molar ratio of Man:Gal:Glc:GlcNH₂ was 68:32:16:2. A trace amount of total phosphorus (0.2%) was found. The predominant amino acids were threonine and serine. The peptide moiety was labeled with [¹⁴C]formaldehyde and the elution of radioactivity was coincident with sugar on gel filtration in the presence of sodium dodecyl sulfate. The peak of radioactivity was retarded on release of galactose by mild acid hydrolysis. These results confirm the sugar-peptide linkage.     

Molecular genetic analysis of a locus required for resistance to antimicrobial peptides in Salmonella typhimurium.

The innate immunity of vertebrates and invertebrates to microbial infection is mediated in part by small cationic peptides with antimicrobial activity. Successful pathogens have evolved mechanisms to withstand the antibiotic activity of these molecules. We have isolated a set of genes from Salmonella typhimurium which are required for virulence and resistance to the antimicrobial peptides melittin and protamine. Sequence analysis of a 5.7 kb segment from the wild-type plasmid conferring resistance to protamine contained five open reading frames: sapA, sapB, sapC, sapD and sapF, organized in an operon structure and transcribed as a 5.3 kb mRNA. SapD and SapF exhibited similarity with the 'ATP binding cassette' family of transporters including the bacterial Opp and SpoOK, involved in the uptake of oligopeptides; the yeast STE6, necessary for the export of a peptide pheromone; and the mammalian mdr, which mediates resistance to chemotherapeutic agents in cancer cells. SapA showed identity with other periplasmic solute binding proteins involved in peptide transport. The SapABCDF system constitutes a novel transporter for enteric bacteria and the first one harboring a periplasmic component with a role in virulence.     

Molecular genetic analysis of the Escherichia coli phoP locus.

We have cloned the Escherichia coli phoP gene, a member of the family of environmentally responsive two-component systems, and found its deduced amino acid sequence to be 93% identical to that of the Salmonella typhimurium homolog, which encodes a major virulence regulator necessary for intramacrophage survival and resistance to cationic peptides of phagocytic cells. The phoP gene was mapped to kilobase 1202 on the Kohara map (25-min region) of the E. coli genome (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987) and found to be transcribed in a counterclockwise direction. Both E. coli and S. typhimurium phoP mutants were more sensitive than their isogenic wild-type strains to the frog-derived antibacterial peptide magainin 2, suggesting a role for PhoP in the response to various stresses in both enteric species.     

Focus: Zoonotic Disease: The Practice of Science as the Pursuit of Knowledge

Choosing what scientific project to pursue is the most important decision that scientists at all levels continually face. Time devoted to a project can further desirable knowledge and advance a career or cost years in lost opportunity. Knowing what to consider before embarking on a specific scientific journey, as well as when to drop a project and change course, offers a way of practicing science that keeps us mindful of what is relevant at a given time and place while preserving our freedom to explore the most exciting findings. This article explores both the pressures that restrict this delicate decision-making process and the processes that scientists can apply to overcome those pressures. Above all else, as it turns out, we must still love the pursuit of knowledge for its own sake – and this love directly impacts our results.     

A Shared Endoplasmic Reticulum-associated Degradation Pathway Involving the EDEM1 Protein for Glycosylated and Nonglycosylated Proteins

Studies of misfolded protein targeting to endoplasmic reticulum-associated degradation (ERAD) have largely focused on glycoproteins, which include the bulk of the secretory proteins. Mechanisms of targeting of nonglycosylated proteins are less clear. Here, we studied three nonglycosylated proteins and analyzed their use of known glycoprotein quality control and ERAD components. Similar to an established glycosylated ERAD substrate, the uncleaved precursor of asialoglycoprotein receptor H2a, its nonglycosylated mutant, makes use of calnexin, EDEM1, and HRD1, but only glycosylated H2a is a substrate for the cytosolic SCF^Fbs2 E3 ubiquitin ligase with lectin activity. Two nonglycosylated BiP substrates, NS-1κ light chain and truncated Igγ heavy chain, interact with the ERAD complex lectins OS-9 and XTP3-B and require EDEM1 for degradation. EDEM1 associates through a region outside of its mannosidase-like domain with the nonglycosylated proteins. Similar to glycosylated substrates, proteasomal inhibition induced accumulation of the nonglycosylated proteins and ERAD machinery in the endoplasmic reticulum-derived quality control compartment. Our results suggest a shared ERAD pathway for glycosylated and nonglycosylated proteins composed of luminal lectin machinery components also capable of protein-protein interactions.     

PERK-dependent compartmentalization of ERAD and unfolded protein response machineries during ER stress

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) activates the ER membrane kinases PERK and IRE1 leading to the unfolded protein response (UPR). We show here that UPR activation triggers PERK and IRE1 segregation from BiP and their sorting with misfolded proteins to the ER-derived quality control compartment (ERQC), a pericentriolar compartment that we had identified previously. PERK phosphorylates translation factor eIF2alpha, which then accumulates on the cytosolic side of the ERQC. Dominant negative PERK or eIF2alpha(S51A) mutants prevent the compartmentalization, whereas eIF2alpha(S51D) mutant, which mimics constitutive phosphorylation, promotes it. This suggests a feedback loop where eIF2alpha phosphorylation causes pericentriolar concentration at the ERQC, which in turn amplifies the UPR. ER-associated degradation (ERAD) is an UPR-dependent process; we also find that ERAD components (Sec61beta, HRD1, p97/VCP, ubiquitin) are recruited to the ERQC, making it a likely site for retrotranslocation. In addition, we show that autophagy, suggested to play a role in elimination of aggregated proteins, is unrelated to protein accumulation in the ERQC.     

The Salmonella SpiC protein targets the mammalian Hook3 protein function to alter cellular trafficking

The Salmonella SpiC protein is secreted into the cytosol of macrophages via a unique type III secretion system that functions intracellularly to translocate proteins across the phagosomal membrane. The SpiC protein is required for survival within macrophages and inhibition of phagosome-lysosome fusion in vivo, and it is sufficient to inhibit endosome-endosome fusion in vitro. Here, we establish that SpiC targets the function of Hook3, a mammalian protein implicated in cellular trafficking. Purified GST-SpiC pulled down Hook3 from murine macrophages, and anti-Hook3 antibodies precipitated SpiC from the cytosol of Salmonella-infected macrophages. Expression of the spiC gene disrupted Golgi morphology in Vero cells and altered the distribution of lysosomes in macrophages, mimicking the phenotype of cells expressing a hook3 dominant-negative mutant. By inactivating Hook3 function, the SpiC protein may alter the lysosome network and prevent phagosome-lysosome fusion.