Four genes encoding different type I signal peptidases are organized in a cluster in Streptomyces lividans TK21

Secretory expression of recombinant small laccase genes in Gram-positive bacteria

BACKGROUND

Laccases are multicopper enzymes that oxidize a wide range of aromatic and non-aromatic compounds in the presence of oxygen. The majority of industrially relevant laccases are derived from fungi and are produced in eukaryotic expression systems such as Pichia pastoris and Saccharomyces cerevisiae. Bacterial laccases for research purposes are mostly produced intracellularly in Escherichia coli, but secretory expression systems are needed for future applications. Bacterial laccases from Streptomyces spp. are of interest for potential industrial applications because of their lignin degrading activities.

RESULTS

In this study, we expressed small laccases genes from Streptomyces coelicolor, Streptomyces viridosporus and Amycolatopsis 75iv2 with their native signal sequences in Gram-positive Bacillus subtilis and Streptomyces lividans host organisms. The extracellular activities of ScLac, SvLac and AmLac expressed in S. lividans reached 1950 ± 99 U/l, 812 ± 57 U/l and 12 ± 1 U/l in the presence of copper supplementation. The secretion of the small laccases was irrespective of the copper supplementation; however, activities upon reconstitution with copper after expression were significantly lower, indicating the importance of copper during laccase production. The production of small laccases in B. subtilis resulted in extracellular activity that was significantly lower than in S. lividans. Unexpectedly, AmLac and ScLac were secreted without their native signal sequences in B. subtilis, indicating that B. subtilis secretes some heterologous proteins via an unknown pathway.

CONCLUSIONS

Small laccases from S. coelicolor, S. viridosporus and Amycolatopsis 75iv2 were secreted in both Gram-positive expression hosts B. subtilis and S. lividans, but the extracellular activities were significantly higher in the latter.

Streptomyces as Microbial Chassis for Heterologous Protein Expression

Heterologous production of recombinant proteins is gaining increasing interest in biotechnology with respect to productivity, scalability, and wide applicability. The members of genus Streptomyces have been proposed as remarkable hosts for heterologous production due to their versatile nature of expressing various secondary metabolite biosynthetic gene clusters and secretory enzymes. However, there are several issues that limit their use, including low yield, difficulty in genetic manipulation, and their complex cellular features. In this review, we summarize rational engineering approaches to optimizing the heterologous production of secondary metabolites and recombinant proteins in Streptomyces species in terms of genetic tool development and chassis construction. Further perspectives on the development of optimal Streptomyces chassis by the design-build-test-learn cycle in systems are suggested, which may increase the availability of secondary metabolites and recombinant proteins.

Bacterial Signal Peptidases

Signal peptidases are the membrane bound enzymes that cleave off the amino-terminal signal peptide from secretory preproteins . There are two types of bacterial signal peptidases . Type I signal peptidase utilizes a serine/lysine catalytic dyad mechanism and is the major signal peptidase in most bacteria. Type II signal peptidase is an aspartic protease specific for prolipoproteins. This chapter will review what is known about the structure, function and mechanism of these unique enzymes.

Protein Secretion in Gram-Positive Bacteria: From Multiple Pathways to Biotechnology

A number of Gram-positive bacteria are important players in industry as producers of a diverse array of economically interesting metabolites and proteins. As discussed in this overview, several Gram-positive bacteria are valuable hosts for the production of heterologous proteins. In contrast to Gram-negative bacteria, proteins secreted by Gram-positive bacteria are released into the culture medium where conditions for correct folding are more appropriate, thus facilitating the isolation and purification of active proteins. Although seven different protein secretion pathways have been identified in Gram-positive bacteria, the majority of heterologous proteins are produced via the general secretion or Sec pathway. Not all proteins are equally well secreted, because heterologous protein production often faces bottlenecks including hampered secretion, susceptibility to proteases, secretion stress, and metabolic burden. These bottlenecks are associated with reduced yields leading to non-marketable products. In this chapter, besides a general overview of the different protein secretion pathways, possible hurdles that may hinder efficient protein secretion are described and attempts to improve yield are discussed including modification of components of the Sec pathway. Attention is also paid to omics-based approaches that may offer a more rational approach to optimize production of heterologous proteins.

ER type I signal peptidase subunit (LmSPC1) is essential for the survival of Locusta migratoria manilensis and affects moulting, feeding, reproduction and embryonic development

The endoplasmic reticulum type I signal peptidase complex (ER SPC) is a conserved enzyme that cleaves the signal peptides of secretory or membrane preproteins. The deletion of this enzyme leads to the accumulation of uncleaved proteins in biomembranes and cell death. However, the physiological functions of ER SPC in insects are not fully understood. Here, a catalytic subunit gene of ER SPC, LmSPC1, was cloned from Locusta migratoria manilensis and its physiological functions were analysed by RNA interference (RNAi). The LmSPC1 open reading frame encoded a protein of 178 amino acids with all five conserved regions of signal peptidases. RNAi-mediated knockdown of LmSPC1 resulted in high mortality. Sixty-nine per cent of dead nymphs died of abnormal moulting, corresponding to decreased activity of moulting fluid protease. Moreover, insects in the RNAi group experienced a decline in food intake, and a decrease in the secretion of total protein and digestive enzymes from midgut tissues to the midgut lumen. Furthermore, the females produced fewer eggs and eggs with disrupted embryogenesis. These results indicate that LmSPC1 is required for the secretion of secretory proteins, affects physiological functions, including moulting, feeding, reproduction and embryonic development, and is essential for survival. Therefore, LmSPC1 may be a potential target for locust control.

Mycoplasma hyopneumoniae type I signal peptidase: expression and evaluation of its diagnostic potential

Type I signal peptidase (SPase I) is a membrane-anchored protease of the general secretory pathway, which is encoded by the sipS gene in Mycoplasma hyopneumoniae, the etiological agent of porcine enzootic pneumonia (PEP). In this study, the expression of the M. hyopneumoniae SPase I (MhSPase I) was analyzed in virulent and avirulent strains, and the recombinant protein (rMhSPase I), expressed in Escherichia coli, was evaluated regarding its potential as an immunodiagnostic antigen. It was demonstrated that the sipS coding DNA sequence (CDS) is most likely part of an operon, being co-transcribed along with four other CDSs. Quantitative reverse transcriptase PCR and immunoblot assays showed that MhSPase I is expressed by all three strains analyzed, with no transcriptional difference, but with evidence of a higher protein level in a pathogenic strain (7422), in comparison to another pathogenic (7448) and a non-pathogenic (J) strain. rMhSPase I was strongly immunogenic for mice, and the MhSPase I antigenicity was confirmed. Polyclonal serum anti-rMhSPase I presented no detectable cross-reaction with Mycoplasma flocculare and Mycoplasma hyorhinis. Moreover, phylogenetic analysis demonstrated a low conservation between MhSPase I and orthologous proteins from other porcine respiratory disease complex-related bacteria, Firmicutes and other Mycoplasma species. The potential of an rMhSPase I-based ELISA for PEP immunodiagnosis was demonstrated. Overall, we investigated the expression of sipS and the encoded MhSPase I in three M. hyopneumoniae strains and showed that this protein is a good antigen for use in PEP serodiagnosis and possibly vaccination, as well as a potential target for antibiotic development.

Isolation and characterization of type I signal peptidase of different malaria parasites

Type I signal peptidases are important membrane-bound serine proteases responsible for the cleavage of the signal peptide of the proteins. These enzymes are unique serine proteases that carry out catalysis using a serine/lysine catalytic dyad. In the present study, we report the isolation of type I signal peptidase from the malaria parasites Plasmodium falciparum, Plasmodium knowlesi, and Plasmodium yoelii and some characterization of type I signal peptidase of Plasmodium falciparum. We show that these enzymes are homologous to signal peptidases from various sources and also contain the conserved boxes present in other type I signal peptidases. The type I signal peptidase from P falciparum is an intron-less and a single-copy gene. The results also show that the enzyme from Plasmodium falciparum is subject to self-cleavage and it has been demonstrated to possess type I signal peptidase activity in E coli preprotein processing in vivo by complementation assay. This study will be helpful in understanding one of the important metabolic pathways “the secretory pathway” in the parasite and should make an important contribution in understanding the complex process of protein targeting in the parasite.

Secretory production of recombinant proteins by Streptomyces

Bacterial systems are widely applied as production platforms for proteins of biopharmaceutical or therapeutic interest and industrial enzymes. Among these prokaryotic systems, streptomycetes are attractive host cells because several strains of these Gram-positive bacteria have a high innate secretion capacity and extensive knowledge on their fermentation is available. A survey of the literature and our own experience suggests that several proteins are secreted to commercially acceptable levels. However, many heterologous proteins, most often of eukaryotic origin, are currently only poorly secreted by this host, indicating the need for further optimization of Streptomyces as a production host. In this review, the considerable efforts and strategies made in recent years aimed at improving streptomycetes as a host for the production of recombinant proteins will be discussed.

Compensatory effect of the minorStreptomyces lividans type I signal peptidases on the SipY major signal peptidase deficiency as determined by extracellular proteome analysis

The developmentally complex bacterium Streptomyces lividans has the ability to produce and secrete a significant amount of protein and possesses four different type I signal peptidase genes (sipW, sipX, sipY and sipZ) that are unusually clustered in its chromosome. 2-DE and subsequent MS of extracellular proteins showed that proteins with typical export signals for type I and type II signal peptidases are the main components of the S. lividans secretome. Secretion of extracellular proteins is severely reduced in a strain deficient in the major type I signal peptidase (SipY). This deficiency was efficiently compensated by complementation with any of the other three signal peptidases as deduced from a comparison of the corresponding 2-D PAGE patterns with that of the wild-type strain.

Surface plasmon resonance-based interaction studies reveal competition of Streptomyces lividans type I signal peptidases for binding preproteins

Type I signal peptidases (SPases) are responsible for the cleavage of signal peptides from secretory proteins.Streptomyces lividanscontains four different SPases, denoted SipW, SipX, SipY and SipZ, having at least some differences in their substrate specificity. In this reportin vitropreprotein binding/processing and protein secretion in single SPase mutants was determined to gain more insight into the substrate specificity of the different SPases and the underlying molecular basis. Results indicated that preproteins do not preferentially bind to a particular SPase, suggesting SPase competition for binding preproteins. This observation, together with the fact that each SPase could process each preprotein tested with a similar efficiency in anin vitroassay, suggested that there is no real specificity in substrate binding and processing, and that they are all actively involved in preprotein processingin vivo. Although this seems to be the case for some proteins tested, high-level secretion of others was clearly dependent on only one particular SPase demonstrating clear differences in substrate preference at thein vivoprocessing level. Hence, these results strongly suggest that there are additional factors other than the cleavage requirements of the enzymes that strongly affect the substrate preference of SPasesin vivo.

Type I signal peptidase: An overview

The signal hypothesis suggests that proteins contain information within their amino acid sequences for protein targeting to the membrane. These distinct targeting sequences are cleaved by specific enzymes known as signal peptidases. There are various type of signal peptidases known such as type I, type II, and type IV. Type I signal peptidases are indispensable enzymes, which catalyze the cleavage of the amino-terminal signal-peptide sequences from preproteins, which are translocated across biological membranes. These enzymes belong to a novel group of serine proteases, which generally utilize a Ser-Lys or Ser-His catalytic dyad instead of the prototypical Ser-His-Asp triad. Despite having no distinct consensus sequence other than a commonly found 'Ala-X-Ala' motif preceding the cleavage site, signal sequences are recognized by type I signal peptidase with high fidelity. Type I signal peptidases have been found in bacteria, archaea, fungi, plants, and animals. In this review, I present an overview of bacterial type I signal peptidases and describe some of their properties in detail.

Type I signal peptidases of Gram-positive bacteria

Proteins that are exported from the cytoplasm to the periplasm and outer membrane of Gram-negative bacteria, or the cell wall and growth medium of Gram-positive bacteria, are generally synthesized as precursors with a cleavable signal peptide. During or shortly after pre-protein translocation across the cytoplasmic membrane, the signal peptide is removed by signal peptidases. Importantly, pre-protein processing by signal peptidases is essential for bacterial growth and viability. This review is focused on the signal peptidases of Gram-positive bacteria, Bacillus and Streptomyces species in particular. Evolutionary concepts, current knowledge of the catalytic mechanism, substrate specificity requirements and structural aspects are addressed. As major insights in signal peptidase function and structure have been obtained from studies on the signal peptidase LepB of Escherichia coli, similarities and differences between this enzyme and known Gram-positive signal peptidases are highlighted. Notably, while the incentive for previous research on Gram-positive signal peptidases was largely based on their role in the biotechnologically important process of protein secretion, present-day interest in these essential enzymes is primarily derived from the idea that they may serve as targets for novel anti-microbials.

A novel sortase, SrtC2, from Streptococcus pyogenes anchors a surface protein containing a QVPTGV motif to the cell wall

The important human pathogen Streptococcus pyogenes (group A streptococcus GAS), requires several surface proteins to interact with its human host. Many of these are covalently linked by a sortase enzyme to the cell wall via a C-terminal LPXTG motif. This motif is followed by a hydrophobic region and charged C terminus, which are thought to retard the protein in the cell membrane to facilitate recognition by the membrane-localized sortase. Previously, we identified two sortase enzymes in GAS. SrtA is found in all GAS strains and anchors most proteins containing LPXTG, while SrtB is present only in some strains and anchors a subset of LPXTG-containing proteins. We now report the presence of a third sortase in most strains of GAS, SrtC. We show that SrtC mediates attachment of a protein with a QVPTGV motif preceding a hydrophobic region and charged tail. We also demonstrate that the QVPTGV sequence is a substrate for anchoring of this protein by SrtC. Furthermore, replacing this motif with LPSTGE, found in the SrtA-anchored M protein of GAS, leads to SrtA-dependent secretion of the protein but does not lead to its anchoring by SrtA. We conclude that srtC encodes a novel sortase that anchors a protein containing a QVPTGV motif to the surface of GAS.

Differential roles of multiple signal peptidases in the virulence of Listeria monocytogenes

Most bacteria contain one type I signal peptidase (Spase I) for cleavage of signal peptides from exported and secreted proteins. Here, we identified a locus encoding three contiguous Spase I genes in the genome of Listeria monocytogenes. The deduced Sip proteins (denoted SipX, SipY and SipZ) are significantly similar to SipS and SipT, the major SPase I proteins of Bacillus subtilis (38% to 44% peptidic identity). We studied the role of these multiple signal peptidases in bacterial pathogenicity by constructing a series of single- and double-chromosomal knock-out mutants. Inactivation of sipX did not affect intracellular multiplication of L. monocytogenes but significantly reduced bacterial virulence (approximately 100-fold). Inactivation of sipZ impaired the secretion of phospholipase C (PC-PLC) and listeriolysin O (LLO), restricted intracellular multiplication and almost abolished virulence (LD(50) of 10(8.3)), inactivation of sipY had no detectable effects. Most importantly, a mutant expressing only SipX was impaired in intracellular survival and strongly attenuated in the mouse (LD(50) of 10(7.2)), whereas, a mutant expressing only SipZ behaved like wild-type EGD in all the assays performed. The data establish that SipX and SipZ perform distinct functions in bacterial pathogenicity and that SipZ is the major Spase I of L. monocytogenes. This work constitutes the first report on the differential role of multiple Spases I in a pathogenic bacterium and suggests a possible post-translational control mechanism of virulence factors expression.

Molecular and functional characterization of type I signal peptidase from Legionella pneumophila

Legionella pneumophila is a facultative intracellular Gram-negative rod-shaped bacterium that has become an important cause of both community-acquired and nosocomial pneumonia. Numerous studies concerning the unravelling of the virulence mechanism of this important pathogen have been initiated. As evidence is now accumulating for the involvement of protein secretion systems in bacterial virulence in general, the type I signal peptidase (LepB) of L. pneumophila was of particular interest. This endopeptidase plays an essential role in the processing of preproteins carrying a typical amino-terminal signal peptide, upon translocation across the cytoplasmic membrane. This paper reports the cloning and the transcriptional analysis of the L. pneumophila lepB gene encoding the type I signal peptidase (SPase). Reverse transcription PCR experiments showed clear lepB expression when L. pneumophila was grown both in culture medium, and also intracellularly in Acanthamoeba castellanii, a natural eukaryotic host of L. pneumophila. In addition, LepB was shown to be encoded by a polycistronic mRNA transcript together with two other proteins, i.e. a LepA homologue and a ribonuclease III homologue. SPase activity of the LepB protein was demonstrated by in vivo complementation analysis in a temperature-sensitive Escherichia coli lepB mutant. Protein sequence and predicted membrane topology were compared to those of leader peptidases of other Gram-negative human pathogens. Most strikingly, a strictly conserved methionine residue in the substrate binding pocket was replaced by a leucine residue, which might influence substrate recognition. Finally it was shown by in vivo experiments that L. pneumophila LepB is a target for (5S,6S)-6-[(R)-acetoxyethyl]-penem-3-carboxylate, a specific inhibitor of type I SPases.

Analysis of type I signal peptidase affinity and specificity for preprotein substrates

Type I signal peptidases (SPases) are membrane-bound endopeptidases responsible for the catalytic cleavage of signal peptides from secretory proteins. Here, we analysed the interaction between a bacterial type I SPase and preprotein substrates using surface plasmon resonance. The use of a home-made biosensor surface based on a mixed self-assembled monolayer of thiols on gold allowed qualitative and kinetic analysis. In vitro binding of purified preproteins to a covalently immobilised bacterial SPase was found to be rather efficient (apparent K(D)=10(-7)-10(-8)M). The signal peptide was shown to be a prerequisite for SPase binding and the nature of the mature part of the preprotein significantly affected SPase binding affinity. The developed biosensor containing immobilised SPase is of great importance for analysis of specificity at substrate binding level and for drug screening. In fact, this is the first report of a membrane protein that was covalently attached to a biosensor surface and that retained binding capacity.

Streptomyces lividans contains a minimal functional signal recognition particle that is involved in protein secretion

The bacterial version of the mammalian signal recognition particle (SRP) is well conserved and essential to all known bacteria. The genes for the Streptomyces lividans SRP components have been cloned and characterized. FtsY resembles the mammalian SRP receptor and the S. lividans SRP consists of Ffh, a homologue of the mammalian SRP54 protein, and scRNA, which is a small size RNA of 82 nt in length. Co-immunoprecipitation studies confirmed that Ffh and scRNA are probably the only components of the S. lividans SRP and that pre-agarase can co-immunoprecipitate with Ffh, suggesting that the SRP is involved in targeting secretory proteins.

Gene function analysis in environmental isolates: the nif regulon of the strict iron oxidizing bacterium Leptospirillum ferrooxidans

A random genomic library from an environmental isolate of the Gram-negative bacterium Leptospirillum ferrooxidans has been printed on a microarray. Gene expression analysis was carried out with total RNA extracted from L. ferrooxidans cultures in the presence or absence of ammonium as nitrogen source under aerobic conditions. Although practically nothing is known about the genome sequence of this bacterium, this approach allowed us the selection and sequencing of only those clones bearing genes that showed an altered expression pattern. By sequence comparison, we have identified most of the genes of nitrogen fixation regulon in L. ferrooxidans, like the nifHDKENX operon, encoding the structural components of Mo-Fe nitrogenase; nifSU-hesB-hscBA-fdx operon, for Fe-S cluster assembly; the amtB gene (ammonium transporter); modA (molybdenum ABC type transporter); some regulatory genes like ntrC, nifA (the specific activator of nif genes); or two glnB-like genes (encoding the PII regulatory protein). Our results show that shotgun DNA microarrays are very powerful tools to accomplish gene expression studies with environmental bacteria whose genome sequence is still unknown, avoiding the time and effort necessary for whole genome sequencing projects.

SipY Is the Streptomyces lividans type I signal peptidase exerting a major effect on protein secretion

Most bacteria contain one type I signal peptidase (SPase) for cleavage of signal peptides from secreted proteins. The developmental complex bacterium Streptomyces lividans has the ability to produce and secrete a significant amount of proteins and has four different type I signal peptidases genes (sipW, sipX, sipY, and sipZ) unusually clustered in its chromosome. Functional analysis of the four SPases was carried out by phenotypical and molecular characterization of the different individual sip mutants. None of the sip genes seemed to be essential for bacterial growth. Analysis of total extracellular proteins indicated that SipY is likely to be the major S. lividans SPase, since the sipY mutant strain is highly deficient in overall protein secretion and extracellular protease production, showing a delayed sporulation phenotype when cultured in solid medium.

In vitro and in vivo self-cleavage of Streptococcus pneumoniae signal peptidase I

We have previously demonstrated that Streptococcus pneumoniae signal peptidase (SPase) I catalyzes a self-cleavage to result in a truncated product, SPase37-204 [Peng, S.B., Wang, L., Moomaw, J., Peery, R.B., Sun, P.M., Johnson, R.B., Lu, J., Treadway, P., Skatrud, P.L. & Wang, Q.M. (2001) J. Bacteriol.183, 621-627]. In this study, we investigated the effect of phospholipid on invitro self-cleavage of S. pneumoniae SPase I. In the presence of phospholipid, the self-cleavage predominantly occurred at one cleavage site between Gly36-His37, whereas the self-cleavage occurred at multiple sites in the absence of phospholipid, and two additional self-cleavage sites, Ala65-His66 and Ala143-Phe144, were identified. All three self-cleavage sites strongly resemble the signal peptide cleavage site and follow the (-1, -3) rule for SPase I recognition. Kinetic analysis demonstrated that self-cleavage is a concentration dependent and intermolecular event, and the activity in the presence of phospholipid is 25-fold higher than that in the absence of phospholipid. Biochemical analysis demonstrated that SPase37-204, the major product of the self-cleavage totally lost activity to cleave its substrates, indicating that the self-cleavage resulted in the inactivation of the enzyme. More importantly, the self-cleavage was demonstrated to be happening in vivo in all the growth phases of S. pneumoniae cells. The bacterial cells keep the active SPase I at the highest level in exponential growth phase, suggesting that the self-cleavage may play an important role in regulating the activity of the enzyme under different conditions.

Physical requirements for in vitro processing of the Streptomyces lividans signal peptidases

The Gram-positive eubacterium Streptomyces lividans contains four chromosomally encoded type I signal peptidases, SipW, SipX, SipY and SipZ, of which all but SipW have an unusual C-terminal membrane anchor. For in vitro characterisation of these signal peptidases, the S. lividans sip genes were expressed in Escherichia coli and the corresponding proteins were purified. The four enzymes had an optimum activity at an alkaline pH, notably pH 8-9 for SipW and SipY and pH 10-11 for SipX and SipZ. In contrast to SipW, the in vitro activities of SipX, SipY and SipZ significantly increased in the presence of detergent. Since none of the S. lividans Sip proteins contains the hydrophobic beta-barrel domain, which in E. coli LepB was proven to be requisite for detergent-dependent in vitro activity, we assume that for detergent dependence, the C-terminal transmembrane anchor can partly substitute for this domain. Finally, all Sip proteins were stimulated by added phospholipids, which strongly suggests that phospholipids play an important role in the catalytic mechanism.

Identification and properties of type I-signal peptidases of Bacillus amyloliquefaciens

The use of Bacillus amyloliquefaciens for enzyme production and its exceptional high protein export capacity initiated this study where the presence and function of multiple type I signal peptidase isoforms was investigated. In addition to type I signal peptidases SipS(ba) [Meijer, W.J.J., de Jong, A., Bea, G., Wisman, A., Tjalsma, H., Venema, G., Bron, S. & van Dijl, J.M. (1995) Mol. Microbiol. 17, 621-631] and SipT(ba) [Hoang, V. & Hofemeister, J. (1995) Biochim. Biophys. Acta 1269, 64-68] which were previously identified, here we present evidence for two other Sip-like genes in B. amyloliquefaciens. Same map positions as well as sequence motifs verified that these genes encode homologues of Bacillus subtilis SipV and SipW. SipU-encoding DNA was not found in B. amyloliquefaciens. SipW-encoding DNA was also found for other Bacillus strains representing different phylogenetic groups, but not for Bacillus stearothermophilus and Thermoactinomyces vulgaris. The absence of these genes, however, could have been overlooked due to sequence diversity. Sequence alignments of 23 known Sip-like proteins from Bacillus origin indicated further branching of the P-group signal peptidases into clusters represented by B. subtilis SipV, SipS-SipT-SipU and B. anthracis Sip3-Sip5 proteins, respectively. Each B. amyloliquefaciens sip(ba) gene was expressed in an Escherichia coli LepBts mutant and tested for genetic complementation of the temperature sensitive (TS) phenotype as well as pre-OmpA processing. Although SipS(ba) as well as SipT(ba) efficiently restored processing of pre-OmpA in E. coli, only SipS(ba) supported growth at TS conditions, indicating functional diversity. Changed properties of the sip(ba) gene disruption mutants, including cell autolysis, motility, sporulation, and nuclease activities, seemed to correlate with specificities and/or localization of B. amyloliquefaciens SipS, SipT and SipV isoforms.

Membrane topology of the Streptomyces lividans type I signal peptidases

Most bacterial membranes contain one or two type I signal peptidases (SPases) for the removal of signal peptides from export proteins. For Streptomyces lividans, four different type I SPases (denoted SipW, SipX, SipY, and SipZ) were previously described. In this communication, we report the experimental determination of the membrane topology of these SPases. A protease protection assay of SPase tendamistat fusions confirmed the presence of the N- as well as the C-terminal transmembrane anchor for SipY. SipX and SipZ have a predicted topology similar to that of SipY. These three S. lividans SPases are currently the only known prokaryotic-type type I SPases of gram-positive bacteria with a C-terminal transmembrane anchor, thereby establishing a new subclass of type I SPases. In contrast, S. lividans SipW contains only the N-terminal transmembrane segment, similar to most type I SPases of gram-positive bacteria. Functional analysis showed that the C-terminal transmembrane anchor of SipY is important to enhance the processing activity, both in vitro as well as in vivo. Moreover, for the S. lividans SPases, a relation seems to exist between the presence or absence of the C-terminal anchor and the relative contributions to the total SPase processing activity in the cell. SipY and SipZ, two SPases with a C-terminal anchor, were shown to be of major importance to the cell. Accordingly, for SipW, missing the C-terminal anchor, a minor role in preprotein processing was found.

The structure and mechanism of bacterial type I signal peptidases. A novel antibiotic target

Type I signal peptidases are essential membrane-bound serine proteases that function to cleave the amino-terminal signal peptide extension from proteins that are translocated across biological membranes. The bacterial signal peptidases are unique serine proteases that utilize a Ser/Lys catalytic dyad mechanism in place of the classical Ser/His/Asp catalytic triad mechanism. They represent a potential novel antibiotic target at the bacterial membrane surface. This review will discuss the bacterial signal peptidases that have been characterized to date, as well as putative signal peptidase sequences that have been recognized via bacterial genome sequencing. We review the investigations into the mechanism of Escherichia coli and Bacillus subtilis signal peptidase, and discuss the results in light of the recent crystal structure of the E. coli signal peptidase in complex with a beta-lactam-type inhibitor. The proposed conserved structural features of Type I signal peptidases give additional insight into the mechanism of this unique enzyme.