Advance online publication:

This section includes articles accepted for publication in Microbial Cell, which have not been released in a regular issue, yet. Please note that the PDF versions of advance publication articles are generally paginated starting with page 1. This does not correspond to the final pagination upon release of the issue it will appear in.

 

Nutrient sensing and cAMP signaling in yeast: G-protein coupled receptor versus transceptor activation of PKA

Griet Van Zeebroeck Liesbeth Demuyser, Zhiqiang Zhang, Ines Cottignie and Johan M. Thevelein

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A major signal transduction pathway regulating cell growth and many associated physiological properties as a function of nutrient availability in the yeast Saccharomyces cerevisiae is the protein kinase A (PKA) pathway. Glucose activation of PKA is mediated by G-protein coupled receptor (GPCR) Gpr1, and secondary messenger cAMP. Other nutrients, including nitrogen, phosphate and sulfate, activate PKA in accordingly-starved cells through nutrient transceptors, but apparently without cAMP signaling. We have now used an optimized EPAC-based fluorescence resonance energy transfer (FRET) sensor to precisely monitor in vivo cAMP levels after nutrient addition. We show that GPCR-mediated glucose activation of PKA is correlated with a rapid transient increase in the cAMP level in vivo, whereas nutrient transceptor-mediated activation by nitrogen, phosphate or sulfate, is not associated with any significant increase in cAMP in vivo. We also demonstrate direct physical interaction between the Gap1 amino acid transceptor and the catalytic subunits of PKA, Tpk1, 2 and 3. In addition, we reveal a conserved consensus motif in the nutrient transceptors that is also present in Bcy1, the regulatory subunit of PKA. This suggests that nutrient transceptor activation of PKA may be mediated by direct release of bound PKA catalytic subunits, triggered by the conformational changes occurring during transport of the substrate by the transceptor. Our results support a model in which nutrient transceptors are evolutionary ancestors of GPCRs, employing a more primitive direct signaling mechanism compared to the indirect cAMP second-messenger signaling mechanism used by GPCRs for activation of PKA.

PDF | Published online: 12/10/2020 | In press

A novel antibacterial strategy: histone and antimicrobial peptide synergy

Leora Duong, Steven P. Gross and Albert Siryaporn

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The rate at which antibiotics are discovered and developed has stagnated; meanwhile, antibacterial resistance continually increases and leads to a plethora of untreatable and deadly infections worldwide. Therefore, there is a critical need to develop new antimicrobial strategies to combat this alarming reality. One approach is to understand natural antimicrobial defense mechanisms that higher-level organisms employ in order to kill bacteria, potentially leading to novel antibiotic therapeutic approaches. Mammalian histones have long been reported to have antibiotic activity, with the first observation of their antibacterial properties reported in 1942. However, there have been doubts about whether histones could truly have any such role in the animal, predominantly based on two issues: they are found in the nucleus (so are not in a position to encounter bacteria), and their antibiotic activity in vitro has been relatively weak in physiological conditions. More recent studies have addressed both sets of concerns. Histones are released from cells as part of neutrophil extracellular traps (NETs) and are thus able to encounter extracellular bacteria. Histones are also present intracellularly in the cytoplasm attached to lipid droplets, positioning them to encounter cytosolic bacteria. Our recent work (Doolin et al., 2020, Nat Commun), which is discussed here, shows that histones have synergistic antimicrobial activities when they are paired with antimicrobial peptides (AMPs), which form pores in bacterial membranes and co-localize with histones in NETs. The work demonstrates that histones enhance AMP-mediated pores, impair bacterial membrane recovery, depolarize the bacterial proton gradient, and enter the bacterial cytoplasm, where they restructure the chromosome and inhibit transcription. Here, we examine potential mechanisms that are responsible for these outcomes.

PDF | Published online: 08/10/2020 | In press

Extracellular vesicles: An emerging platform in gram-positive bacteria

Swagata Bose, Shifu Aggarwal, Durg Vijai Singh and Narottam Acharya

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Extracellular vesicles (EV), also known as membrane vesicles, are produced as an end product of secretion by both pathogenic and non-pathogenic bacteria. Several reports suggest that archaea, gram-negative bacteria, and eukaryotic cells secrete membrane vesicles as a means for cell-free intercellular communication. EVs influence intercellular communication by transferring a myriad of biomolecules including genetic information. Also, EVs have been implicated in many phenomena such as stress response, intercellular competition, lateral gene transfer, and pathogenicity. However, the cellular process of secreting EVs in gram-positive bacteria is less studied. A notion with the thick cell-walled microbes such as gram-positive bacteria is that the EV release is impossible among them. The role of gram-positive EVs in health and diseases is being studied gradually. Being nano-sized, the EVs from gram-positive bacteria carry a diversity of cargo compounds that have a role in bacterial competition, survival, invasion, host immune evasion, and infection. In this review, we summarise the current understanding of the EVs produced by gram-positive bacteria. Also, we discuss the functional aspects of these components while comparing them with gram-negative bacteria.

PDF | Published online: 05/10/2020 | In press

Structural insights into the architecture and assembly of eukaryotic flagella

Narcis-Adrian Petriman and Esben Lorentzen

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Cilia and flagella are slender projections found on most eukaryotic cells including unicellular organisms such as Chlamydomonas, Trypanosoma and Tetrahymena, where they serve motility and signaling functions. The cilium is a large molecular machine consisting of hundreds of different proteins that are trafficked into the organelle to organize a repetitive microtubule-based axoneme. Several recent studies took advantage of improved cryo-EM methodology to unravel the high-resolution structures of ciliary complexes. These include the recently reported purification and structure determination of axonemal doublet microtubules from the green algae Chlamydomonas reinhardtii, which allows for the modeling of more than 30 associated protein factors to provide deep molecular insight into the architecture and repetitive nature of doublet microtubules. In addition, we will review several recent contributions that dissect the structure and function of ciliary trafficking complexes that ferry structural and signaling components between the cell body and the cilium organelle.

PDF | Published online: 21/09/2020 | In press

Novobiocin inhibits membrane synthesis and vacuole formation of Enterococcus faecalis protoplasts

Rintaro Tsuchikado, Satoshi Kami, Sawako Takahashi and Hiromi Nishida

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We demonstrate that plasma membrane biosynthesis and vacuole formation require DNA replication in Enterococcus faecalis protoplasts. The replication inhibitor novobiocin inhibited not only DNA replication but also cell enlargement (plasma membrane biosynthesis) and vacuole formation during the enlargement of the E. faecalis protoplasts. After novobiocin treatment prior to vacuole formation, the cell size of E. faecalis protoplasts was limited to 6 μm in diameter and the cells lacked vacuoles. When novobiocin was added after vacuole formation, E. faecalis protoplasts grew with vacuole enlargement; after novobiocin removal, protoplasts were enlarged again. Although cell size distribution of the protoplasts was similar following the 24 h and 48 h novobiocin treatments, after 72 h of novobiocin treatment there was a greater number of smaller sized protoplasts, suggesting that extended novobiocin treatment may inhibit the re-enlargement of E. faecalis protoplasts after novobiocin removal. Our findings demonstrate that novobiocin can control the enlargement of E. faecalis protoplasts due to inhibition of DNA replication.

PDF | Published online: 10/08/2020 | In press

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