Impact of membrane fusion and proteolysis on SpoIIQ dynamics and interaction with SpoIIIAH

The small acid-soluble proteins of Clostridioides difficile regulate sporulation in a SpoIVB2-dependent manner

Clostridioides difficile is a pathogen whose transmission relies on the formation of dormant endospores. Spores are highly resilient forms of bacteria that resist environmental and chemical insults. In recent work, we found that C. difficile SspA and SspB, two small acid-soluble proteins (SASPs), protect spores from UV damage and, interestingly, are necessary for the formation of mature spores. Here, we build upon this finding and show that C. difficile sspA and sspB are required for the formation of the spore cortex layer. Moreover, using an EMS mutagenesis selection strategy, we identified mutations that suppressed the defect in sporulation of C. difficile SASP mutants. Many of these strains contained mutations in CDR20291_0714 (spoIVB2) revealing a connection between the SpoIVB2 protease and the SASPs in the sporulation pathway. This work builds upon the hypothesis that the small acid-soluble proteins can regulate gene expression.

Cleavage of an engulfment peptidoglycan hydrolase by a sporulation signature protease in Clostridioides difficile

In the model organism Bacillus subtilis, a signaling protease produced in the forespore, SpoIVB, is essential for the activation of the sigma factor σK, which is produced in the mother cell as an inactive pro-protein, pro-σK. SpoIVB has a second function essential to sporulation, most likely during cortex synthesis. The cortex is composed of peptidoglycan (PG) and is essential for the spore's heat resistance and dormancy. Surprisingly, the genome of the intestinal pathogen Clostridioides difficile, in which σK is produced without a pro-sequence, encodes two SpoIVB paralogs, SpoIVB1 and SpoIVB2. Here, we show that spoIVB1 is dispensable for sporulation, while a spoIVB2 in-frame deletion mutant fails to produce heat-resistant spores. The spoIVB2 mutant enters sporulation, undergoes asymmetric division, and completes engulfment of the forespore by the mother cell but fails to synthesize the spore cortex. We show that SpoIIP, a PG hydrolase and part of the engulfasome, the machinery essential for engulfment, is cleaved by SpoIVB2 into an inactive form. Within the engulfasome, the SpoIIP amidase activity generates the substrates for the SpoIID lytic transglycosylase. Thus, following engulfment completion, the cleavage and inactivation of SpoIIP by SpoIVB2 curtails the engulfasome hydrolytic activity, at a time when synthesis of the spore cortex peptidoglycan begins. SpoIVB2 is also required for normal late gene expression in the forespore by a currently unknown mechanism. Together, these observations suggest a role for SpoIVB2 in coordinating late morphological and gene expression events between the forespore and the mother cell.

The small acid-soluble proteins of Clostridioides difficile regulate sporulation in a SpoIVB2-dependent manner

Clostridioides difficile is a pathogen whose transmission relies on the formation of dormant endospores. Spores are highly resilient forms of bacteria that resist environmental and chemical insults. In recent work, we found that C. difficile SspA and SspB, two small acid-soluble proteins (SASPs), protect spores from UV damage and, interestingly, are necessary for the formation of mature spores. Here, we build upon this finding and show that C. difficile sspA and sspB are required for the formation of the spore cortex layer. Moreover, using an EMS mutagenesis selection strategy, we identified mutations that suppressed the defect in sporulation of C. difficile SASP mutants. Many of these strains contained mutations in CDR20291_0714 (spoIVB2) revealing a connection between the SpoIVB2 protease and the SASPs in the sporulation pathway. This work builds upon the hypothesis that the small acid-soluble proteins can regulate gene expression.

Structural, Metabolic and Evolutionary Comparison of Bacterial Endospore and Exospore Formation

Sporulation is a specialized developmental program employed by a diverse set of bacteria which culminates in the formation of dormant cells displaying increased resilience to stressors. This represents a major survival strategy for bacteria facing harsh environmental conditions, including nutrient limitation, heat, desiccation, and exposure to antimicrobial compounds. Through dispersal to new environments via biotic or abiotic factors, sporulation provides a means for disseminating genetic material and promotes encounters with preferable environments thus promoting environmental selection. Several types of bacterial sporulation have been characterized, each involving numerous morphological changes regulated and performed by non-homologous pathways. Despite their likely independent evolutionary origins, all known modes of sporulation are typically triggered by limited nutrients and require extensive membrane and peptidoglycan remodeling. While distinct modes of sporulation have been observed in diverse species, two major types are at the forefront of understanding the role of sporulation in human health, and microbial population dynamics and survival. Here, we outline endospore and exospore formation by members of the phyla Firmicutes and Actinobacteria, respectively. Using recent advances in molecular and structural biology, we point to the regulatory, genetic, and morphological differences unique to endo- and exospore formation, discuss shared characteristics that contribute to the enhanced environmental survival of spores and, finally, cover the evolutionary aspects of sporulation that contribute to bacterial species diversification.

Channels modestly impact compartment-specific ATP levels during Bacillus subtilis sporulation and a rise in the mother cell ATP level is not necessary for Pro-σK cleavage

Starvation of Bacillus subtilis initiates endosporulation involving formation of mother cell (MC) and forespore (FS) compartments. During engulfment, the MC membrane migrates around the FS and protein channels connect the two compartments. The channels are necessary for postengulfment FS gene expression, which relieves inhibition of SpoIVFB, an intramembrane protease that cleaves Pro-σ^K , releasing σ^K into the MC. SpoIVFB has an ATP-binding domain exposed to the MC cytoplasm, but the role of ATP in regulating Pro-σ^K cleavage has been unclear, as has the impact of the channels on MC and FS ATP levels. Using luciferase produced separately in each compartment to measure relative ATP concentrations during sporulation, we found that the MC ATP concentration rises about twofold coincident with increasing cleavage of Pro-σ^K , and the FS ATP concentration does not decline. Mutants lacking a channel protein or defective in channel protein turnover exhibited modest and varied effects on ATP levels, which suggested that low ATP concentration does not explain the lack of postengulfment FS gene expression in channel mutants. Furthermore, a rise in the MC ATP level was not necessary for Pro-σ^K cleavage by SpoIVFB, based on analysis of mutants that bypass the need for relief of SpoIVFB inhibition.

A Hybrid Secretion System Facilitates Bacterial Sporulation: A Structural Perspective

Bacteria employ a number of dedicated secretion systems to export proteins to the extracellular environment. Several of these comprise large complexes that assemble in and around the bacterial membrane(s) to form specialized channels through which only selected proteins are actively delivered. Although typically associated with bacterial pathogenicity, a specialized variant of these secretion systems has been proposed to play a central part in bacterial sporulation, a primitive protective process that allows starving cells to form spores that survive in extreme environments. Following asymmetric division, the mother cell engulfs the forespore, leaving it surrounded by two bilayer membranes. During the engulfment process an essential channel apparatus is thought to cross both membranes to create a direct conduit between the mother cell and forespore. At least nine proteins are essential for channel formation, including SpoIIQ under forespore control and the eight SpoIIIA proteins (SpoIIIAA to -AH) under mother cell control. Presumed to form a core channel complex, several of these proteins share similarity with components of Gram-negative bacterial secretion systems, including the type II, III, and IV secretion systems and the flagellum. Based on these similarities it has been suggested that the sporulation channel represents a hybrid, secretion-like transport machinery. Recently, in-depth biochemical and structural characterization of the individual channel components accompanied by in vivo studies has further reinforced this model. Here we review and discuss these recent studies and suggest an updated model for the unique sporulation channel apparatus architecture.

Evidence that regulation of intramembrane proteolysis is mediated by substrate gating during sporulation in Bacillus subtilis

During the morphological process of sporulation in Bacillus subtilis two adjacent daughter cells (called the mother cell and forespore) follow different programs of gene expression that are linked to each other by signal transduction pathways. At a late stage in development, a signaling pathway emanating from the forespore triggers the proteolytic activation of the mother cell transcription factor σ^K. Cleavage of pro-σ^K to its mature and active form is catalyzed by the intramembrane cleaving metalloprotease SpoIVFB (B), a Site-2 Protease (S2P) family member. B is held inactive by two mother-cell membrane proteins SpoIVFA (A) and BofA. Activation of pro-σ^K processing requires a site-1 signaling protease SpoIVB (IVB) that is secreted from the forespore into the space between the two cells. IVB cleaves the extracellular domain of A but how this cleavage activates intramembrane proteolysis has remained unclear. Structural studies of the Methanocaldococcus jannaschii S2P homolog identified closed (substrate-occluded) and open (substrate-accessible) conformations of the protease, but the biological relevance of these conformations has not been established. Here, using co-immunoprecipitation and fluorescence microscopy, we show that stable association between the membrane-embedded protease and its substrate requires IVB signaling. We further show that the cytoplasmic cystathionine-β-synthase (CBS) domain of the B protease is not critical for this interaction or for pro-σ^K processing, suggesting the IVB-dependent interaction site is in the membrane protease domain. Finally, we provide evidence that the B protease domain adopts both open and closed conformations in vivo. Collectively, our data support a substrate-gating model in which IVB-dependent cleavage of A on one side of the membrane triggers a conformational change in the membrane-embedded protease from a closed to an open state allowing pro-σ^K access to the caged interior of the protease.

Regulated Intramembrane Proteolysis is a broadly conserved mechanism for transducing information across lipid bilayers. In these signaling pathways a protease on one side of the membrane triggers the activation of a membrane-embedded protease that cleaves its substrate within or adjacent to the cytoplasmic face of the membrane. Site-2 metalloproteases (S2P) are the most commonly used intramembrane cleaving proteases in these pathways but the mechanism by which cleavage on one side of the membrane triggers intramembrane proteolysis remains poorly understood. Here, we provide evidence for a substrate-gating model in which an extracellular signaling protease triggers a conformational change in a S2P family member from a closed to an open conformation allowing its substrate access to the catalytic center of the enzyme.

GerM is required to assemble the basal platform of the SpoIIIA-SpoIIQ transenvelope complex during sporulation in Bacillus subtilis

Sporulating Bacillus subtilis cells assemble a multimeric membrane complex connecting the mother cell and developing spore that is required to maintain forespore differentiation. An early step in the assembly of this transenvelope complex (called the A-Q complex) is an interaction between the extracellular domains of the forespore membrane protein SpoIIQ and the mother cell membrane protein SpoIIIAH. This interaction provides a platform onto which the remaining components of the complex assemble and also functions as an anchor for cell-cell signalling and morphogenetic proteins involved in spore development. SpoIIQ is required to recruit SpoIIIAH to the sporulation septum on the mother cell side; however, the mechanism by which SpoIIQ specifically localizes to the septal membranes on the forespore side has remained enigmatic. Here, we identify GerM, a lipoprotein previously implicated in spore germination, as the missing factor required for SpoIIQ localization. Our data indicate that GerM and SpoIIIAH, derived from the mother cell, and SpoIIQ, from the forespore, have reciprocal localization dependencies suggesting they constitute a tripartite platform for the assembly of the A-Q complex and a hub for the localization of mother cell and forespore proteins.

A Membrane-Embedded Amino Acid Couples the SpoIIQ Channel Protein to Anti-Sigma Factor Transcriptional Repression during Bacillus subtilis Sporulation

SpoIIQ is an essential component of a channel connecting the developing forespore to the adjacent mother cell during Bacillus subtilis sporulation. This channel is generally required for late gene expression in the forespore, including that directed by the late-acting sigma factor σ(G) Here, we present evidence that SpoIIQ also participates in a previously unknown gene regulatory circuit that specifically represses expression of the gene encoding the anti-sigma factor CsfB, a potent inhibitor of σ(G) The csfB gene is ordinarily transcribed in the forespore only by the early-acting sigma factor σ(F) However, in a mutant lacking the highly conserved SpoIIQ transmembrane amino acid Tyr-28, csfB was also aberrantly transcribed later by σ(G), the very target of CsfB inhibition. This regulation of csfB by SpoIIQ Tyr-28 is specific, given that the expression of other σ(F)-dependent genes was unaffected. Moreover, we identified a conserved element within the csfB promoter region that is both necessary and sufficient for SpoIIQ Tyr-28-mediated inhibition. These results indicate that SpoIIQ is a bifunctional protein that not only generally promotes σ(G)activity in the forespore as a channel component but also specifically maximizes σ(G)activity as part of a gene regulatory circuit that represses σ(G)-dependent expression of its own inhibitor, CsfB. Finally, we demonstrate that SpoIIQ Tyr-28 is required for the proper localization and stability of the SpoIIE phosphatase, raising the possibility that these two multifunctional proteins cooperate to fine-tune developmental gene expression in the forespore at late times.

IMPORTANCE

Cellular development is orchestrated by gene regulatory networks that activate or repress developmental genes at the right time and place. Late gene expression in the developing Bacillus subtilis spore is directed by the alternative sigma factor σ(G) The activity of σ(G)requires a channel apparatus through which the adjacent mother cell provides substrates that generally support gene expression. Here we report that the channel protein SpoIIQ also specifically maximizes σ(G)activity as part of a previously unknown regulatory circuit that prevents σ(G)from activating transcription of the gene encoding its own inhibitor, the anti-sigma factor CsfB. The discovery of this regulatory circuit significantly expands our understanding of the gene regulatory network controlling late gene expression in the developing B. subtilis spore.

Complex Formed between Intramembrane Metalloprotease SpoIVFB and Its Substrate, Pro-σK

Intramembrane metalloproteases (IMMPs) are conserved from bacteria to humans and control many important signaling pathways, but little is known about how IMMPs interact with their substrates. SpoIVFB is an IMMP that cleaves Pro-σ(K) during Bacillus subtilis endospore formation. When catalytically inactive SpoIVFB was coexpressed with C-terminally truncated Pro-σ(K)(1-126) (which can be cleaved by active SpoIVFB) in Escherichia coli, the substrate dramatically improved solubilization of the enzyme from membranes with mild detergents. Both the Pro(1-20) and σ(K)(21-126) parts contributed to improving SpoIVFB solubilization from membranes, but only the σ(K) part was needed to form a stable complex with SpoIVFB in a pulldown assay. The last 10 residues of SpoIVFB were required for improved solubilization from membranes by Pro-σ(K)(1-126) and for normal interaction with the substrate. The inactive SpoIVFB·Pro-σ(K)(1-126)-His6 complex was stable during affinity purification and gel filtration chromatography. Disulfide cross-linking of the purified complex indicated that it resembled the complex formed in vivo Ion mobility-mass spectrometry analysis resulted in an observed mass consistent with a 4:2 SpoIVFB·Pro-σ(K)(1-126)-His6 complex. Stepwise photobleaching of SpoIVFB fused to a fluorescent protein supported the notion that the enzyme is tetrameric during B. subtilis sporulation. The results provide the first evidence that an IMMP acts as a tetramer, give new insights into how SpoIVFB interacts with its substrate, and lay the foundation for further biochemical analysis of the enzyme·substrate complex and future structural studies.

The SpoIIQ-SpoIIIAH complex of Clostridium difficile controls forespore engulfment and late stages of gene expression and spore morphogenesis

Engulfment of the forespore by the mother cell is a universal feature of endosporulation. In Bacillus subtilis, the forespore protein SpoIIQ and the mother cell protein SpoIIIAH form a channel, essential for endosporulation, through which the developing spore is nurtured. The two proteins also form a backup system for engulfment. Unlike in B. subtilis, SpoIIQ of Clostridium difficile has intact LytM zinc‐binding motifs. We show that spoIIQ or spoIIIAH deletion mutants of C. difficile result in anomalous engulfment, and that disruption of the SpoIIQ LytM domain via a single amino acid substitution (H120S) impairs engulfment differently. SpoIIQ and SpoIIQ^H120S interact with SpoIIIAH throughout engulfment. SpoIIQ, but not SpoIIQ^H120S, binds Zn²⁺, and metal absence alters the SpoIIQ‐SpoIIIAH complex in vitro. Possibly, SpoIIQ^H120S supports normal engulfment in some cells but not a second function of the complex, required following engulfment completion. We show that cells of the spoIIQ or spoIIIAH mutants that complete engulfment are impaired in post‐engulfment, forespore and mother cell‐specific gene expression, suggesting a channel‐like function. Both engulfment and a channel‐like function may be ancestral functions of SpoIIQ‐SpoIIIAH while the requirement for engulfment was alleviated through the emergence of redundant mechanisms in B. subtilis and related organisms.

A mother cell-to-forespore channel: current understanding and future challenges

Formation of endospores allows some bacteria to survive extreme nutrient limitation. The resulting dormant cell, the spore, persists in the environment and is highly resistant to physical and chemical stresses. During spore formation, cells divide asymmetrically and the mother cell engulfs the developing spore, encasing it within a double membrane and isolating it from the medium. Communication between mother cell and isolated forespore involves a specialised connection system that allows nurturing of the forespore and continued macromolecular synthesis, required to finalise spore maturation. Here, we review current understanding of this feeding channel formed by a forespore protein, SpoIIQ, and a mother cell protein, SpoIIIAH, in the model organism Bacillus subtilis and the important human pathogen Clostridium difficile. We also analyse the presence of this channel across endospore-forming bacteria and highlight the main questions still remaining.

Regulated proteolysis in bacterial development

Bacteria use proteases to control three types of events temporally and spatially during the processes of morphological development. These events are the destruction of regulatory proteins, activation of regulatory proteins, and production of signals. While some of these events are entirely cytoplasmic, others involve intramembrane proteolysis of a substrate, transmembrane signaling, or secretion. In some cases, multiple proteolytic events are organized into pathways, for example turnover of a regulatory protein activates a protease that generates a signal. We review well-studied and emerging examples and identify recurring themes and important questions for future research. We focus primarily on paradigms learned from studies of model organisms, but we note connections to regulated proteolytic events that govern bacterial adaptation, biofilm formation and disassembly, and pathogenesis.

Biochemical and structural insights into intramembrane metalloprotease mechanisms

Intramembrane metalloproteases are nearly ubiquitous in living organisms and they function in diverse processes ranging from cholesterol homeostasis and the unfolded protein response in humans to sporulation, stress responses, and virulence of bacteria. Understanding how these enzymes function in membranes is a challenge of fundamental interest with potential applications if modulators can be devised. Progress is described toward a mechanistic understanding, based primarily on molecular genetic and biochemical studies of human S2P and bacterial SpoIVFB and RseP, and on the structure of the membrane domain of an archaeal enzyme. Conserved features of the enzymes appear to include transmembrane helices and loops around the active site zinc ion, which may be near the membrane surface. Extramembrane domains such as PDZ (PSD-95, DLG, ZO-1) or CBS (cystathionine-β-synthase) domains govern substrate access to the active site, but several different mechanisms of access and cleavage site selection can be envisioned, which might differ depending on the substrate and the enzyme. More work is needed to distinguish between these mechanisms, both for enzymes that have been relatively well-studied, and for enzymes lacking PDZ and CBS domains, which have not been studied. This article is part of a Special Issue entitled: Intramembrane Proteases.

The SpoIIQ landmark protein has different requirements for septal localization and immobilization

Bacillus subtilis sporulation depends on the forespore membrane protein SpoIIQ, which interacts with the mother cell protein SpoIIIAH at the septum to localize other sporulation proteins. It has remained unclear how SpoIIQ localizes. We demonstrate that localization of SpoIIQ is achieved by two pathways: SpoIIIAH and the SpoIID, SpoIIM, SpoIIP engulfment proteins. SpoIIQ shows diffuse localization only in a mutant lacking both pathways. Super-resolution imaging shows that in the absence of SpoIIIAH, SpoIIQ forms fewer, slightly larger foci than in wild type. Surprisingly, photobleaching experiments demonstrate that, although SpoIIQ localizes without SpoIIIAH, it is no longer immobilized, and is therefore able to exchange subunits within a localized pool. SpoIIQ mobility is further increased by the additional absence of the engulfment proteins. However an enzymatically inactive SpoIID protein immobilizes SpoIIQ even in the absence of SpoIIIAH, indicating that complete septal thinning is not required for SpoIIQ localization. This suggests that SpoIIQ interacts with both SpoIIIAH and the engulfment proteins or their peptidoglycan cleavage products. They further demonstrate that apparently normal localization of a protein without a binding partner can mask dramatic alterations in protein mobility. We speculate that SpoIIQ assembles foci along the path defined by engulfment proteins degrading peptidoglycan.

Genomic determinants of sporulation in Bacilli and Clostridia: towards the minimal set of sporulation-specific genes

Three classes of low-G+C Gram-positive bacteria (Firmicutes), Bacilli, Clostridia and Negativicutes, include numerous members that are capable of producing heat-resistant endospores. Spore-forming firmicutes include many environmentally important organisms, such as insect pathogens and cellulose-degrading industrial strains, as well as human pathogens responsible for such diseases as anthrax, botulism, gas gangrene and tetanus. In the best-studied model organism Bacillus subtilis, sporulation involves over 500 genes, many of which are conserved among other bacilli and clostridia. This work aimed to define the genomic requirements for sporulation through an analysis of the presence of sporulation genes in various firmicutes, including those with smaller genomes than B. subtilis. Cultivable spore-formers were found to have genomes larger than 2300 kb and encompass over 2150 protein-coding genes of which 60 are orthologues of genes that are apparently essential for sporulation in B. subtilis. Clostridial spore-formers lack, among others, spoIIB, sda, spoVID and safA genes and have non-orthologous displacements of spoIIQ and spoIVFA, suggesting substantial differences between bacilli and clostridia in the engulfment and spore coat formation steps. Many B. subtilis sporulation genes, particularly those encoding small acid-soluble spore proteins and spore coat proteins, were found only in the family Bacillaceae, or even in a subset of Bacillus spp. Phylogenetic profiles of sporulation genes, compiled in this work, confirm the presence of a common sporulation gene core, but also illuminate the diversity of the sporulation processes within various lineages. These profiles should help further experimental studies of uncharacterized widespread sporulation genes, which would ultimately allow delineation of the minimal set(s) of sporulation-specific genes in Bacilli and Clostridia.

Holin triggering in real time

During λ infections, the holin S105 accumulates harmlessly in the membrane until, at an allele-specific time, suddenly triggering to form irregular holes of unprecedented size (>300 nm), releasing the endolysin from the cytoplasm, resulting in lysis within seconds. Here we used a functional S105-GFP chimera and real-time deconvolution fluorescence microscopy to show that the S105-GFP fusion accumulated in a uniformly distributed fashion, until suddenly, within 1 min, it formed aggregates, or rafts, at the time of lethal triggering. Moreover, the isogenic fusion to a nonlethal S105 mutant remained uniformly distributed, whereas a fusion to an early-lysing mutant showed early triggering and early raft formation. Protein accumulation rates of the WT, early, and nonlethal alleles were identical. Fluorescence recovery after photobleaching (FRAP) revealed that the nonlethal mutant and untriggered WT hybrids were highly mobile in the membrane, whereas the WT raft was essentially immobile. Finally, an antiholin allele, S105(ΔTMD1)-mcherryfp, in the product of which the S105 sequence deleted for the first transmembrane domain was fused to mCherryFP. This hybrid retained full antiholin activity, in that it blocked lethal hole formation by the S105-GFP fusion, accumulated uniformly throughout the host membrane and prevented the S105-GFP protein from forming rafts. These findings suggest that phage lysis occurs when the holin reaches a critical concentration and nucleates to form rafts, analogous to the initiation of purple membrane formation after the induction of bacteriorhodopsin in halobacteria. This model for holin function may be relevant for processes in mammalian cells, including the release of nonenveloped viruses and apoptosis.

A LytM domain dictates the localization of proteins to the mother cell-forespore interface during bacterial endospore formation

A large number of proteins are known to reside at specific subcellular locations in bacterial cells. However, the molecular mechanisms by which many of these proteins are anchored at these locations remains unclear. During endospore formation in Bacillus subtilis, several integral membrane proteins are located specifically at the interface of the two adjacent cells of the developing sporangium, the mother cell and forespore. The mother cell membrane protein SpoIIIAH recognizes the cell-cell interface through an interaction with the forespore membrane protein SpoIIQ, and then the other proteins are positioned there by the SpoIIIAH-SpoIIQ complex. In this study, we investigated the molecular mechanisms underlying the formation of the SpoIIIAH-SpoIIQ complex. Using gel filtration chromatography and isothermal titration calorimetry, we measured the binding parameters that characterize the SpoIIIAH-SpoIIQ interaction in vitro. We also demonstrated that the interaction of SpoIIIAH and SpoIIQ is governed by their YscJ and degenerate LytM domains, respectively. Therefore, the LytM domain of SpoIIQ provides the positional cue that dictates the localization of mother cell membrane proteins to the mother cell-forespore interface.

SpoIID-mediated peptidoglycan degradation is required throughout engulfment during Bacillus subtilis sporulation

SpoIID is a membrane-anchored enzyme that degrades peptidoglycan and is essential for engulfment and sporulation in Bacillus subtilis. SpoIID is targeted to the sporulation septum, where it interacts with two other proteins required for engulfment: SpoIIP and SpoIIM. We changed conserved amino acids in SpoIID to alanine to determine whether there was a correlation between the effect of each substitution on the in vivo and in vitro activities of SpoIID. We identified one amino acid substitution, E88A, that eliminated peptidoglycan degradation activity and one, D210A, that reduced it, as well as two substitutions that destabilized the protein in B. subtilis (R106A and K203A). Using these mutants, we show that the peptidoglycan degradation activity of SpoIID is required for the first step of engulfment (septal thinning), as well as throughout membrane migration, and we show that SpoIID levels are substantially above the minimum required for engulfment. The inactive mutant E88A shows increased septal localization compared to the wild type, suggesting that the degradation cycle of the SpoIID/SpoIIP complex is accompanied by the activity-dependent release of SpoIID from the complex and subsequent rebinding. This mutant is also capable of moving SpoIIP across the sporulation septum, suggesting that SpoIID binding, but not peptidoglycan degradation activity, is needed for relocalization of SpoIIP. Finally, the mutant with reduced activity (D210A) causes uneven engulfment and time-lapse microscopy indicates that the fastest-moving membrane arm has greater concentrations of SpoIIP than the slower-moving arm, demonstrating a correlation between SpoIIP protein levels and the rate of membrane migration.

A channel connecting the mother cell and forespore during bacterial endospore formation

At an early stage during Bacillus subtilis endospore development the bacterium divides asymmetrically to produce two daughter cells. The smaller cell (forespore) differentiates into the endospore, while the larger cell (mother cell) becomes a terminally differentiated cell that nurtures the developing forespore. During development the mother cell engulfs the forespore to produce a protoplast, surrounded by two bilayer membranes, which separate it from the cytoplasm of the mother cell. The activation of sigma(G), which drives late gene expression in the forespore, follows forespore engulfment and requires expression of the spoIIIA locus in the mother cell. One of the spoIIIA-encoded proteins SpoIIIAH is targeted specifically to the membrane surrounding the forespore, through an interaction of its C-terminal extracellular domain with the C-terminal extracellular domain of the forespore membrane protein SpoIIQ. We identified a homologous relationship between the C-terminal domain of SpoIIIAH and the YscJ/FliF protein family, members of which form multimeric rings involved in type III secretion systems and flagella. If SpoIIIAH forms a similar ring structure, it may also form a channel between the mother cell and forespore membranes. To test this hypothesis we developed a compartmentalized biotinylation assay, which we used to show that the C-terminal extracellular domain of SpoIIIAH is accessible to enzymatic modification from the forespore cytoplasm. These and other results lead us to suggest that SpoIIIAH forms part of a channel between the forespore and mother cell that is required for the activation of sigma(G).

SpoIVB and CtpB are both forespore signals in the activation of the sporulation transcription factor sigmaK in Bacillus subtilis

The proteolytic activation of the mother cell transcription factor pro-sigma(K) is controlled by a signal transduction pathway during sporulation in the bacterium Bacillus subtilis. The pro-sigma(K) processing enzyme SpoIVFB, a membrane-embedded metalloprotease, is held inactive by two other integral membrane proteins, SpoIVFA and BofA, in the mother cell membrane that surrounds the forespore. Two signaling serine proteases, SpoIVB and CtpB, trigger pro-sigma(K) processing by cleaving the regulatory protein SpoIVFA. The SpoIVB signal is absolutely required to activate pro-sigma(K) processing and is derived from the forespore compartment. CtpB is necessary for the proper timing of sigma(K) activation and was thought to be a mother cell signal. Here, we show that the ctpB gene is expressed in both the mother cell and forespore compartments but that synthesis in the forespore under the control of sigma(G) is both necessary and sufficient for the proper timing of pro-sigma(K) processing. We further show that SpoIVB cleaves CtpB in vitro and in vivo but that this cleavage does not appear to be necessary for CtpB activation. Thus, both signaling proteins are made in the forespore and independently target the same regulatory protein.

Perturbations to engulfment trigger a degradative response that prevents cell-cell signalling during sporulation in Bacillus subtilis

During sporulation in Bacillus subtilis, the mother cell membranes migrate around the forespore in a phagocytic-like process called engulfment. Developmental gene expression requires the successful completion of this key morphological event. Here we show that perturbations to engulfment block the accumulation of proteins secreted into the space between the mother cell and forespore membranes. Our data support a model in which engulfment defects cause the proteolytic clearance of these secreted proteins. Importantly, we show that this degradative response is reversible; once proper engulfment is restored, secreted proteins again accumulate. In particular, we have found that the forespore signalling protein SpoIVB fails to accumulate when engulfment is impaired and, as a result, late mother cell gene expression under the control of sigma(K) is blocked. If engulfment is restored, SpoIVB accumulates and cell-cell signalling resumes. Thus, this degradative pathway functions like a developmental checkpoint ensuring that mother cell gene expression does not commence unless morphogenesis proceeds normally.