Morphogenesis of bacillus spore surfaces

Spores produced by bacilli are encased in a proteinaceous multilayered coat and, in some species (including Bacillus anthracis), further surrounded by a glycoprotein-containing exosporium. To characterize bacillus spore surface morphology and to identify proteins that direct formation of coat surface features, we used atomic-force microscopy (AFM) to image the surfaces of wild-type and mutant spores of Bacillus subtilis, as well as the spore surfaces of Bacillus cereus 569 and the Sterne strain of Bacillus anthracis. This analysis revealed that the coat surfaces in these strains are populated by a series of bumps ranging between 7 and 40 nm in diameter, depending on the species. Furthermore, a series of ridges encircled the spore, most of which were oriented along the long axis of the spore. The structures of these ridges differ sufficiently between species to permit species-specific identification. We propose that ridges are formed early in spore formation, when the spore volume likely decreases, and that when the spore swells during germination the ridges unfold. AFM analysis of a set of B. subtilis coat protein gene mutants revealed three coat proteins with roles in coat surface morphology: CotA, CotB, and CotE. Our data indicate novel roles for CotA and CotB in ridge pattern formation. Taken together, these results are consistent with the view that the coat is not inert. Rather, the coat is a dynamic structure that accommodates changes in spore volume.     

Functional analysis of the Bacillus subtilis morphogenetic spore coat protein CotE

Bacterial spores are surrounded by a multilayered proteinaceous shell called the coat. In Bacillus subtilis, a coat protein called CotE guides the assembly of a major subset of coat proteins. To understand how CotE carries out its role in coat morphogenesis, we subjected its gene to mutagenesis and studied the effects of altered versions of CotE on coat formation. We identified regions within the C-terminal 28 amino acids that direct the deposition of the coat proteins CotA, CotB, CotG, CotSA, CotS and 35 kDa and 49 kDa proteins likely to be the spore proteins CotR (formerly known as YvdO) and YaaH respectively. The timing and genetic dependency of CotR accumulation are consistent with control of its gene by sigmaK and GerE. In addition, we identified a 35-amino-acid internal region involved in targeting of CotE to the forespore. Finally, we found that sequences within this 35-amino-acid region as well as within an 18-amino-acid stretch in the N-terminus of CotE direct the formation of CotE multimers, most probably homooligomers. These results suggest that: (i) most interactions between CotE and the coat proteins assembled under CotE control take place at the CotE C-terminus; (ii) an internal region of CotE connects it with the forespore surface; and (iii) interactions between CotE molecules depend on residues within an 18-amino-acid region in the N-terminal half of CotE.     

Surface layers of Clostridium difficile endospores

Clostridium difficile is an important human pathogen and one where the primary cause of disease is due to the transmission of spores. We have investigated the proteins found in the outer coat layers of C. difficile spores of pathogenic strain 630 (CD630). Five coat proteins, CotA, CotB, CotCB, CotD, and CotE, were shown to be expressed on the outer coat layers of the spore. We demonstrate that purified spores carry catalase, peroxiredoxin, and chitinase activity and that this activity correlates with the predicted functions of three spore coat proteins identified here, CotCB, CotD, and CotE. CotCB and CotD are putative manganese catalases, and CotE is a novel bifunctional protein with peroxiredoxin activity at its amino terminus and chitinase activity at its carboxy terminus. These enzymes could play an important role in coat assembly by polymerizing protein monomers in the coat. CotE, in addition to a role in macromolecular degradation, could play an important role in inflammation, and this may be of direct relevance to the development of the gastrointestinal symptoms that accompany C. difficile infection. Although specific enzyme activity has not yet been assigned to the proteins identified here, this work provides the first detailed study of the C. difficile spore coat.     

Functional regions of the Bacillus subtilis spore coat morphogenetic protein CotE

The Bacillus subtilis spore is encased in a resilient, multilayered proteinaceous shell, called the coat, that protects it from the environment. A 181-amino-acid coat protein called CotE assembles into the coat early in spore formation and plays a morphogenetic role in the assembly of the coat's outer layer. We have used a series of mutant alleles of cotE to identify regions involved in outer coat protein assembly. We found that the insertion of a 10-amino-acid epitope, between amino acids 178 and 179 of CotE, reduced or prevented the assembly of several spore coat proteins, including, most likely, CotG and CotB. The removal of 9 or 23 of the C-terminal-most amino acids resulted in an unusually thin outer coat from which a larger set of spore proteins was missing. In contrast, the removal of 37 amino acids from the C terminus, as well as other alterations between amino acids 4 and 160, resulted in the absence of a detectable outer coat but did not prevent localization of CotE to the forespore. These results indicate that changes in the C-terminal 23 amino acids of CotE and in the remainder of the protein have different consequences for outer coat protein assembly.     

A novel small protein of Bacillus subtilis involved in spore germination and spore coat assembly

Two small genes named sscA (previously yhzE) and orf-62, located in the prsA-yhaK intergenic region of the Bacillus subtilis genome, were transcribed by SigK and GerE in the mother cells during the later stages of sporulation. The SscA-FLAG fusion protein was produced from T(5) of sporulation and incorporated into mature spores. sscA mutant spores exhibited poor germination, and Tricine-SDS-PAGE analysis showed that the coat protein profile of the mutant differed from that of the wild type. Bands corresponding to proteins at 59, 36, 5, and 3 kDa were reduced in the sscA null mutant. Western blot analysis of anti-CotB and anti-CotG antibodies showed reductions of the proteins at 59 kDa and 36 kDa in the sscA mutant spores. These proteins correspond to CotB and CotG. By immunoblot analysis of an anti-CotH antibody, we also observed that CotH was markedly reduced in the sscA mutant spores. It appears that SscA is a novel spore protein involved in the assembly of several components of the spore coat, including CotB, CotG, and CotH, and is associated with spore germination.     

Protein filaments may initiate the assembly of the Bacillus subtilis spore coat.

The Bacillus subtilis spore coat consists of three morphological layers: a diffuse undercoat, a striated inner coat and a densely staining outer coat. These layers are comprised of at least 15 polypeptides and the absence of one in particular, CotE, had extensive pleiotropic effects. Only a partial inner coat was present on the spores which were lysozyme-sensitive. The initial rate of germination of these spores was the same as for the wild type but the overall optical density decrease was greater apparently due to the loss of the incomplete spore coat from germinated spores. Suppressors of the lysozyme-sensitive phenotype had some outer coat proteins restored as well as some novel minor polypeptides. These spores still lacked an undercoat and germinated as did those produced by the cotE deletion strain. The CotE protein was synthesized starting at stage II-III of sporulation, long before the appearance of the coat on spores at stage IV-V. Despite its apparent hydrophilic properties, this protein was present in the crude insoluble fraction from sporulating cells. CotE was not solubilized by high or low ionic strength buffers not by detergents used for the solubilization of membrane proteins. Either 8 M urea or 6 M guanidine HC1 was required and dialysis against a low ionic strength buffer resulted in aggregation into long, sticky filaments. Both the CotE and CotT spore coat proteins appeared to be necessary for the formation of these filaments. Each of these proteins contains sequences related to a bovine intermediate filament protein so their interaction could result in an analogous structure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered arrangement of proteins in the spore coat of a germination mutant of Bacillus subtilis

Spores produced by a mutant of Bacillus subtilis were slow to develop their resistance properties during sporulation, and were slower to germinate than were wild-type spores. The coat protein composition of the mutant spores, as analysed by SDS-PAGE, was similar to that of the wild-type spores. However, one of the proteins (mol. wt 12000) which is normally present in the outer-most layers of mature wild-type spores and which is surface-exposed, was assembled abnormally into the coat of the mutant spores and not surface-exposed. The mutation responsible for this phenotype (spo-520) has been mapped between pheA and leuB on the B. subtilis chromosome, and was 47% cotransformable with leuB16. This mutation, and three others closely linked to it, define a new sporulation locus, spoVIB, which is involved in spore coat assembly. The phenotype of the mutant(s) supports the contention that spore germination and resistance properties may be determined by the assembly of the coat.     

Morphogenetic proteins SpoVID and SafA form a complex during assembly of the Bacillus subtilis spore coat

During endospore formation in Bacillus subtilis, over two dozen polypeptides are assembled into a multilayered structure known as the spore coat, which protects the cortex peptidoglycan (PG) and permits efficient germination. In the initial stages of coat assembly a protein known as CotE forms a ring around the forespore. A second morphogenetic protein, SpoVID, is required for maintenance of the CotE ring during the later stages, when most of proteins are assembled into the coat. Here, we report on a protein that appears to associate with SpoVID during the early stage of coat assembly. This protein, which we call SafA for SpoVID-associated factor A, is encoded by a locus previously known as yrbA. We confirmed the results of a previous study that showed safA mutant spores have defective coats which are missing several proteins. We have extended these studies with the finding that SafA and SpoVID were coimmunoprecipitated by anti-SafA or anti-SpoVID antiserum from whole-cell extracts 3 and 4 h after the onset of sporulation. Therefore, SafA may associate with SpoVID during the early stage of coat assembly. We used immunogold electron microscopy to localize SafA and found it in the cortex, near the interface with the coat in mature spores. SafA appears to have a modular design. The C-terminal region of SafA is similar to those of several inner spore coat proteins. The N-terminal region contains a sequence that is conserved among proteins that associate with the cell wall. This motif in the N-terminal region may target SafA to the PG-containing regions of the developing spore.     

SpoVID guides SafA to the spore coat in Bacillus subtilis

Bacteria assemble complex structures by targeting proteins to specific subcellular locations. The protein coat that encases Bacillus subtilis spores is an example of a structure that requires coordinated targeting and assembly of more than 24 polypeptides. The earliest stages of coat assembly require the action of three morphogenetic proteins: SpoIVA, CotE, and SpoVID. In the first steps, a basement layer of SpoIVA forms around the surface of the forespore, guiding the subsequent positioning of a ring of CotE protein about 75 nm from the forespore surface. SpoVID localizes near the forespore membrane where it functions to maintain the integrity of the CotE ring and to anchor the nascent coat to the underlying spore structures. However, it is not known which spore coat proteins interact directly with SpoVID. In this study we examined the interaction between SpoVID and another spore coat protein, SafA, in vivo using the yeast two-hybrid system and in vitro. We found evidence that SpoVID and SafA directly interact and that SafA interacts with itself. Immunofluorescence microscopy showed that SafA localized around the forespore early during coat assembly and that this localization of SafA was dependent on SpoVID. Moreover, targeting of SafA to the forespore was also dependent on SpoIVA, as was targeting of SpoVID to the forespore. We suggest that the localization of SafA to the spore coat requires direct interaction with SpoVID.     

Morphogenesis of the Bacillus anthracis spore

Bacillus spp. and Clostridium spp. form a specialized cell type, called a spore, during a multistep differentiation process that is initiated in response to starvation. Spores are protected by a morphologically complex protein coat. The Bacillus anthracis coat is of particular interest because the spore is the infective particle of anthrax. We determined the roles of several B. anthracis orthologues of Bacillus subtilis coat protein genes in spore assembly and virulence. One of these, cotE, has a striking function in B. anthracis: it guides the assembly of the exosporium, an outer structure encasing B. anthracis but not B. subtilis spores. However, CotE has only a modest role in coat protein assembly, in contrast to the B. subtilis orthologue. cotE mutant spores are fully virulent in animal models, indicating that the exosporium is dispensable for infection, at least in the context of a cotE mutation. This has implications for both the pathophysiology of the disease and next-generation therapeutics. CotH, which directs the assembly of an important subset of coat proteins in B. subtilis, also directs coat protein deposition in B. anthracis. Additionally, however, in B. anthracis, CotH effects germination; in its absence, more spores germinate than in the wild type. We also found that SpoIVA has a critical role in directing the assembly of the coat and exosporium to an area around the forespore. This function is very similar to that of the B. subtilis orthologue, which directs the assembly of the coat to the forespore. These results show that while B. anthracis and B. subtilis rely on a core of conserved morphogenetic proteins to guide coat formation, these proteins may also be important for species-specific differences in coat morphology. We further hypothesize that variations in conserved morphogenetic coat proteins may play roles in taxonomic variation among species.     

CotA of Bacillus subtilis is a copper-dependent laccase

The spore coat protein CotA of Bacillus subtilis displays similarities with multicopper oxidases, including manganese oxidases and laccases. B. subtilis is able to oxidize manganese, but neither CotA nor other sporulation proteins are involved. We demonstrate that CotA is a laccase. Syringaldazine, a specific substrate of laccases, reacted with wild-type spores but not with DeltacotA spores. CotA may participate in the biosynthesis of the brown spore pigment, which appears to be a melanin-like product and to protect against UV light.     

The yabG gene of Bacillus subtilis encodes a sporulation specific protease which is involved in the processing of several spore coat proteins

The synthesis and proteolysis of the spore coat proteins, SpoIVA and YrbA, of Bacillus subtilis were analyzed using antisera. Almost no intact full-length proteins of either type were extracted from wild-type spores, while yabG mutant spores contained intact SpoIVA and YrbA proteins. We purified recombinant YrbA and YabG proteins from Escherichia coli transformants and found that YrbA was cleaved to the smaller moiety in the presence of YabG in vitro. These observations indicate that YabG is a protease involved in the proteolysis and maturation of SpoIVA and YrbA proteins, conserved with the cortex and/or coat assembly by B. subtilis.     

Role of the spore coat layers in Bacillus subtilis spore resistance to hydrogen peroxide, artificial UV-C, UV-B, and solar UV radiation

Spores of Bacillus subtilis possess a thick protein coat that consists of an electron-dense outer coat layer and a lamellalike inner coat layer. The spore coat has been shown to confer resistance to lysozyme and other sporicidal substances. In this study, spore coat-defective mutants of B. subtilis (containing the gerE36 and/or cotE::cat mutation) were used to study the relative contributions of spore coat layers to spore resistance to hydrogen peroxide (H(2)O(2)) and various artificial and solar UV treatments. Spores of strains carrying mutations in gerE and/or cotE were very sensitive to lysozyme and to 5% H(2)O(2), as were chemically decoated spores of the wild-type parental strain. Spores of all coat-defective strains were as resistant to 254-nm UV-C radiation as wild-type spores were. Spores possessing the gerE36 mutation were significantly more sensitive to artificial UV-B and solar UV radiation than wild-type spores were. In contrast, spores of strains possessing the cotE::cat mutation were significantly more resistant to all of the UV treatments used than wild-type spores were. Spores of strains carrying both the gerE36 and cotE::cat mutations behaved like gerE36 mutant spores. Our results indicate that the spore coat, particularly the inner coat layer, plays a role in spore resistance to environmentally relevant UV wavelengths.     

Sporulation Temperature Reveals a Requirement for CotE in the Assembly of both the Coat and Exosporium Layers of Bacillus cereus Spores

The Bacillus cereus spore surface layers consist of a coat surrounded by an exosporium. We investigated the interplay between the sporulation temperature and the CotE morphogenetic protein in the assembly of the surface layers of B. cereus ATCC 14579 spores and on the resulting spore properties. The cotE deletion affects the coat and exosporium composition of the spores formed both at the suboptimal temperature of 20°C and at the optimal growth temperature of 37°C. Transmission electron microscopy revealed that ΔcotE spores had a fragmented and detached exosporium when formed at 37°C. However, when produced at 20°C, ΔcotE spores showed defects in both coat and exosporium attachment and were susceptible to lysozyme and mutanolysin. Thus, CotE has a role in the assembly of both the coat and exosporium, which is more important during sporulation at 20°C. CotE was more represented in extracts from spores formed at 20°C than at 37°C, suggesting that increased synthesis of the protein is required to maintain proper assembly of spore surface layers at the former temperature. ΔcotE spores formed at either sporulation temperature were impaired in inosine-triggered germination and resistance to UV-C and H₂O₂ and were less hydrophobic than wild-type (WT) spores but had a higher resistance to wet heat. While underscoring the role of CotE in the assembly of B. cereus spore surface layers, our study also suggests a contribution of the protein to functional properties of additional spore structures. Moreover, it also suggests a complex relationship between the function of a spore morphogenetic protein and environmental factors such as the temperature during spore formation.     

Study of the interactions between the key spore coat morphogenetic proteins CotE and SpoVID

The capability of Bacillus subtilis spores to withstand extreme environmental conditions is thought to be conferred especially by their outermost proteinaceous protective layer, called the spore coat. Of the over 70 proteins that form the spore coat, only a small subset of them affect its morphogenesis, they are referred to as morphogenetic proteins. In this study we investigated the interaction between two spore coat morphogenetic proteins SpoVID and CotE. SpoVID is involved in the process of spore surface encirclement by individual coat proteins, these include CotE, which controls the assembly of the outer coat layer. Both proteins were proposed to be recruited to a common protein scaffold, but their direct association has not been previously shown. Here we studied the interactions between CotE and SpoVID in vitro for the first time by using molecule recognition force spectroscopy, which allows the detection of piconewton forces between conjugated biological pairs and also facilitates the investigation of dynamic processes. The most probable CotE–CotE unbinding force was 49.4 ± 0.1 pN at a loading rate of 3.16 × 10³ pN/s while that of SpoVID–CotE was 26.5 ± 0.6 pN at a loading rate of 7.8 × 10² pN/s. We further analyzed the interactions with the bacterial two hybrid system and pull-down experiments, which also indicate that SpoVID interacts directly with CotE. In combination with the previously identified direct contacts among SpoIVA, SpoVID and SafA, our data imply that the physical association of key morphogenetic proteins forms a basic skeleton where other coat proteins could be attached.     

Disruption of a sigma factor gene, sigF, affects an intermediate stage of spore pigment production in Streptomyces aureofaciens

The Streptomyces aureofaciens sigF gene encodes a sigma factor. By integrative transformation, via double cross-over, a stable null mutant of sigF gene was obtained. This mutation appeared to have no obvious effect on vegetative growth, but affected the late stage of spore maturation. Microscopic examination showed that spores were deformed, and spore wall was thinner, compared with the wild-type spores. The spore pigment of sigF mutant was green, compared to wild-type grey-pink spore pigmentation. The plasmid-born wild-type sigF gene complemented the mutation after transformation of the mutant strain.     

Localization of Proteins to Different Layers and Regions of Bacillus subtilis Spore Coats

Bacterial spores are encased in a multilayered proteinaceous shell known as the coat. In Bacillus subtilis, over 50 proteins are involved in spore coat assembly but the locations of these proteins in the spore coat are poorly understood. Here, we describe methods to estimate the positions of protein fusions to fluorescent proteins in the spore coat by using fluorescence microscopy. Our investigation suggested that CotD, CotF, CotT, GerQ, YaaH, YeeK, YmaG, YsnD, and YxeE are present in the inner coat and that CotA, CotB, CotC, and YtxO reside in the outer coat. In addition, CotZ and CgeA appeared in the outermost layer of the spore coat and were more abundant at the mother cell proximal pole of the forespore, whereas CotA and CotC were more abundant at the mother cell distal pole of the forespore. These polar localizations were observed both in sporangia prior to the release of the forespore from the mother cell and in mature spores after release. Moreover, CotB was observed at the middle of the spore as a ring- or spiral-like structure. Formation of this structure required cotG expression. Thus, we conclude not only that the spore coat is a multilayered assembly but also that it exhibits uneven spatial distribution of particular proteins.Proper localization and assembly of proteins in cells and subcellular structures are essential features of living organisms. Complex protein assemblies, including ribosomes, flagella, and the cytokinetic machinery, play important roles in bacteria (26, 27, 40). Studying how these complex structures are formed is a fundamental theme in molecular biology. In this work, we developed a method to analyze one of the most complex bacterial protein assemblies: the spore coat of Bacillus subtilis.Sporulation of B. subtilis is initiated in response to nutrient limitation, and it involves a highly ordered program of gene expression and morphological change (33, 42). The first morphological change of sporulation is the appearance of an asymmetrically positioned septum that divides the cell into a larger mother cell and a smaller forespore. Next, the mother cell membrane migrates around the forespore membrane during a phagocytosis-like process called engulfment. The completion of engulfment involves fusion of the mother cell membrane to pinch off the forespore within the mother cell. Compartment-specific gene expression brings about maturation of the spore and its release upon lysis of the mother cell (reviewed in reference 19). Mature spores remain viable during long periods of starvation and are resistant to heat, toxic chemicals, lytic enzymes, and other factors capable of damaging vegetative cells (30). Spores germinate and resume growth when nutrients become available (32).The outer portions of Bacillus spores consist of a cortex, a spore coat layer, and in some cases, an exosporium. The cortex, a thick layer of peptidoglycan, is deposited between the inner and the outer membranes of the forespore, and it is responsible for maintaining the highly dehydrated state of the core, thereby contributing to the extreme dormancy and heat resistance of spores. Spore coat assembly involves the deposition of at least 50 protein species (12, 21, 24) into two major layers: an electron-dense outer layer, called the outer coat, and a less electron-dense inner layer with a lamellar appearance, called the inner coat (50). These layers provide a protective barrier against bactericidal enzymes and chemicals, such as lysozyme and organic solvents (30). Although disruption of any one gene encoding a spore coat protein typically has little or no effect on spore resistance, morphology, or germination, a few proteins, referred to as morphogenetic proteins, play central roles in the assembly of the spore coat (7, 10, 13). One of the morphogenetic proteins, CotE, is located between the inner and outer coats and directs the assembly of most or all of the outer coat proteins and also a few of the inner coat proteins (2, 9, 17, 25, 52). The locations of CotE, CotS, and SpoIVA in the spore coat were determined previously by immunoelectron microscopy (9, 43). CotA, CotB, CotC, and CotG were shown to be externally exposed on the surface of the spore by single-molecule recognition force spectroscopy or antibody accessibility (15, 18, 45, 28). However, the positions of most of the spore coat proteins in the coat have not been determined experimentally, although provisional assignments were made based largely on the control of assembly into the coat by CotE (17). In this study, we developed methods to estimate the positions of proteins in the spore coat layers by using fluorescence microscopy analysis of coat protein-fluorescent protein fusions, with resolution that allowed us to distinguish between the inner and outer coats. In addition, we discovered an asymmetric spatial distribution of four spore coat proteins and a ring- or spiral-like structure of CotB. These observations suggest that spore coat assembly is more intricate than previously appreciated.