Summary Although prokaryotes ordinarily undergo binary fission to produce two identical daughter cells, some are able to undergo alternative developmental pathways that produce daughter cells of distinct cell morphology and fate. One such example is a developmental program called sporulation in the bacterium Bacillus subtilis, which occurs under conditions of environmental stress. Sporulation has long been used as a model system to help elucidate basic processes of developmental biology including transcription regulation, intercellular signaling, membrane remodeling, protein localization, and cell fate determination. This review highlights some of the recent work that has been done to further understand prokaryotic cell differentiation during sporulation and its potential applications.

Pyruvate kinase isoform M2 (PKM2) plays an important role in the growth and metabolic reprogramming of cancer cells in stress conditions. Here, we report that SAICAR (succinylaminoimidazolecarboxamide ribose-5′-phosphate, an intermediate of the de novo purine nucleotide synthesis pathway) specifically stimulates PKM2. Upon glucose starvation, cellular SAICAR concentration increases in an oscillatory manner and stimulates PKM2 activity in cancer cells. Changes in SAICAR levels in cancer cells alter cellular energy level, glucose uptake, and lactate production. The SAICAR-PKM2 interaction also promotes cancer cell survival in glucose-limited conditions. SAICAR accumulation is not observed in normal adult epithelial cells or lung fibroblasts regardless of glucose conditions. This allosteric regulation may explain how cancer cells coordinate different metabolic pathways to optimize their growth in the nutrient-limited conditions commonly observed in the tumor microenvironment.

In bacteria, certain shape-sensing proteins localize to differently curved membranes. During sporulation in Bacillus subtilis, the only convex (positively curved) surface in the cell is the forespore, an approximately spherical internal organelle. Previously, we demonstrated that SpoVM localizes to the forespore by preferentially adsorbing onto slightly convex membranes. Here, we used NMR and molecular dynamics simulations of SpoVM and a localization mutant (SpoVM P9A ) to reveal that SpoVM's atypical amphipathic α-helix inserts deeply into the membrane and interacts extensively with acyl chains to sense packing differences in differently curved membranes. Based on binding to spherical supported lipid bilayers and Monte Carlo simulations, we hypothesize that SpoVM's membrane insertion, along with potential cooperative interactions with other SpoVM molecules in the lipid bilayer, drives its preferential localization onto slightly convex membranes. Such a mechanism, which is distinct from that used by high curvature-sensing proteins, may be widely conserved for the localization of proteins onto the surface of cellular organelles.ArfGAP1 | SpoIVA | DivIVA | curvature sensing | lipid packing

SUMMARY Recent discoveries of regulated cell death in bacteria have led to speculation about possible benefits that apoptosis-like pathways may confer to single-celled organisms. However, establishing how these pathways provide increased ecological fitness has remained difficult to determine. Here, we report a pathway in Bacillus subtilis in which regulated cell death maintains the fidelity of sporulation through selective removal of cells that misassemble the spore envelope. The spore envelope, which protects the dormant spore’s genome from environmental insults, uses the protein SpoIVA as a scaffold for assembly. We found that disrupting envelope assembly activates a cell death pathway wherein the small protein CmpA acts as an adaptor to the AAA+ ClpXP protease to degrade SpoIVA, thereby halting sporulation and resulting in lysis of defective sporulating cells. We propose that removal of unfit cells from a population of terminally differentiating cells protects against evolutionary deterioration, and ultimately loss, of the sporulation program.

The type VII secretion system (T7SS) of Staphylococcus aureus is a multiprotein complex dedicated to the export of several virulence factors during host infection. This virulence pathway plays a key role in promoting bacterial survival and the long-term persistence of staphylococcal abscess communities. The expression of the T7SS is activated by bacterial interaction with host tissues including blood serum, nasal secretions, and pulmonary surfactant. In this work we identify the major stimulatory factors as host-specific cisunsaturated fatty acids. Increased T7SS expression requires host fatty acid incorporation into bacterial biosynthetic pathways by the S. aureus fatty acid kinase (FAK) complex, and FakA is required for virulence. The incorporated cis-unsaturated fatty acids decrease S. aureus membrane fluidity, and these altered membrane dynamics are partially responsible for T7SS activation. These data define a molecular mechanism by which S. aureus cells sense the host environment and implement appropriate virulence pathways.type VII secretion | Staphylococcus aureus | fatty acids | virulence

Summary Mature spores of the bacterium Bacillus subtilis are encased by two concentric shells: an inner shell (the ‘cortex’), made of peptidoglycan; and an outer proteinaceous shell (the ‘coat’), whose basement layer is anchored to the surface of the developing spore via a 26‐amino‐acid‐long protein called SpoVM. During sporulation, initiation of cortex assembly depends on the successful initiation of coat assembly, but the mechanisms that co‐ordinate the morphogenesis of both structures are largely unknown. Here, we describe a sporulation pathway involving SpoVM and a 37‐amino‐acid‐long protein named ‘CmpA’ that is encoded by a previously un‐annotated gene and is expressed under control of two sporulation‐specific transcription factors (σE and SpoIIID). CmpA localized to the surface of the developing spore and deletion of cmpA resulted in cells progressing through the sporulation programme more quickly. Overproduction of CmpA did not affect normal growth or cell division, but delayed entry into sporulation and abrogated cortex assembly. In those cells that had successfully initiated coat assembly, CmpA was removed by a post‐translational mechanism, presumably in order to overcome the sporulation inhibition it imposed. We propose a model in which CmpA participates in a developmental checkpoint that ensures the proper orchestration of coat and cortex morphogenesis by repressing cortex assembly until coat assembly successfully initiates.

Relapse-prone, poor prognosis neuroblastoma is frequently characterized by deletion of chr1p36 where tumor suppressor gene KIF1Bβ resides. Interestingly, many 1p36-positive patients failed to express KIF1Bβ protein. Since altered cellular redox status has been reported to be involved in cell death and protein modification, we investigated the relationship between reactive oxygen species (ROS) and KIF1Bβ. Here, we showed that wild-type KIF1Bβ protein expression positively correlates with superoxide (O2 −) and total ROS levels in neuroblastoma cells, unlike apoptotic loss-of-function KIF1Bβ mutants. Overexpression of KIF1Bβ apoptotic domain variants increases total ROS and, specifically O2 −, whereas knockdown of endogenous KIF1Bβ decreases ROS and O2 −. Interestingly, O2 − increases KIF1Bβ protein expression, independent of the proteasomal degradation pathway. Scavenging O2 − or ROS decreases KIF1Bβ protein expression and subsequent apoptosis. Moreover, treatment with investigational redox compound Gliotoxin increases O2 −, KIF1Bβ protein expression, apoptosis and colony formation inhibition. Overall, our findings suggest that ROS and O2 − may be important downstream effectors of KIF1Bβ-mediated apoptosis. Subsequently, O2 − produced may increase KIF1Bβ protein expression in a positive feedback mechanism. Therefore, ROS and, specifically O2 −, may be critical regulators of KIF1Bβ-mediated apoptosis and its protein expression in neuroblastoma.

Remodeling of membranes by fission or fusion has been extensively studied in eukaryotes, but proteins directly responsible for mediating such events in bacteria have not been discovered. A recent report identified a protein in Bacillus subtilis that exploits an affinity for a specific lipid to drive membrane fission during sporulation.