Cytokinesis signals truncation of the PodJ polarity factor by a cell cycle-regulated protease

Expression of hurP, a gene encoding a prospective site 2 protease, is essential for heme-dependent induction of bhuR in Bordetella bronchiseptica

Expression of the hurIR bhuRSTUV heme utilization locus in Bordetella bronchiseptica is coordinately controlled by the global iron-dependent regulator Fur and the extracytoplasmic function sigma factor HurI. Activation of HurI requires transduction of a heme-dependent signal via HurI, HurR, and BhuR, a three-component heme-dependent regulatory system. In silico searches of the B. bronchiseptica genome to identify other genes that encode additional participants in this heme-dependent regulatory cascade revealed hurP, an open reading frame encoding a polypeptide with homology to (i) RseP, a site 2 protease (S2P) of Escherichia coli required for modifying the cytoplasmic membrane protein RseA, and (ii) YaeL, an S2P of Vibrio cholerae required for modification of the cytoplasmic membrane protein TcpP. A mutant of B. bronchiseptica defective for hurP was incapable of regulating expression of BhuR in a heme-dependent manner. Furthermore, the hurP mutant was unable to utilize hemin as a sole source of nutrient Fe. These defects in hemin utilization and heme-dependent induction of BhuR were restored when recombinant hurP (or recombinant rseP) was introduced into the mutant. Introduction of hurP into a yaeL mutant of V. cholerae also complemented its S2P defect. These data provided strong evidence that protease activity and cleavage site recognition was conserved in HurP, RseP, and YaeL. The data are consistent with a model in which HurP functionally modifies HurR, a sigma factor regulator that is essential for heme-dependent induction of bhuR.

Spatial complexity and control of a bacterial cell cycle

A major breakthrough in understanding the bacterial cell cycle is the discovery that bacteria exhibit a high degree of intracellular organization. Chromosomal loci and many protein complexes are positioned at particular subcellular sites. In this review, we examine recently discovered control mechanisms that make use of dynamically localized protein complexes to orchestrate the Caulobacter crescentus cell cycle. Protein localization, notably of signal transduction proteins, chromosome partition proteins, and proteases, serves to coordinate cell division with chromosome replication and cell differentiation. The developmental fate of daughter cells is decided before completion of cytokinesis, via the early establishment of cell polarity by the distribution of activated signaling proteins, bacterial cytoskeleton, and landmark proteins.

Exploration into the spatial and temporal mechanisms of bacterial polarity

The recognition of bacterial asymmetry is not new: the first high-resolution microscopy studies revealed that bacteria come in a multitude of shapes and sometimes carry asymmetrically localized external structures such as flagella on the cell surface. Even so, the idea that bacteria could have an inherent overall polarity, which affects not only their outer appearance but also many of their vital processes, has only recently been appreciated. In this review, we focus on recent advances in our understanding of the molecular mechanisms underlying the establishment of polarized functions and cell polarity in bacteria.

YpdC determines site-1 degradation in regulated intramembrane proteolysis of the RsiW anti-sigma factor of Bacillus subtilis

Genes of Bacillus subtilis controlled by the alternative extracytoplasmic function family sigma factor sigmaW constitute an antibiosis regulon. Its activity is modulated by RsiW, a transmembrane anti-sigma factor that sequesters and inactivates sigmaW. Upon a stress signal, RsiW is degraded by a mechanism of regulated intramembrane proteolysis. To identify genes which influence RsiW degradation, a transposon screen with a reporter fusion of the green fluorescent protein to RsiW was performed. Among several gene loci identified, the ypdC (prsW) gene displayed a strong effect on RsiW stability. In a ypdC null mutant, induction of sigmaW-controlled genes is abolished and site-1 proteolysis of RsiW is completely blocked. Transcriptional analysis revealed that ypdC is a monocistronic gene, and the defect of sigmaW induction of the null mutant was complemented by ectopically integrated ypdC under xylose control. Orthologues of YpdC can be found in a variety of different bacteria. Its membrane topology was analysed by alkaline phosphatase fusions, revealing that YpdC contains five transmembrane segments and two larger extracytoplasmic loops. In the first loop, two invariantly conserved glutamate residues can be found. In an Escherichia coli system, the cloned ypdC is the only determinant of efficient degradation of RsiW; however, YpdC does not display plain similarities to known proteases, suggesting that it either controls the activity of site-1 proteolysis of RsiW or represents a new type of protease.

Regulated intramembrane proteolysis of FtsL protein and the control of cell division in Bacillus subtilis

The small bitopic division protein FtsL is an essential part of the division machinery (divisome) in most eubacteria. In Bacillus subtilis FtsL is a highly unstable protein and the turnover has been implicated in regulation of division in response to DNA damage. N-terminal deletions and a domain swap experiment identified the short cytoplasmic domain of FtsL as being required for instability. We then identified a zinc metalloprotease, YluC, required for turnover, and likely sequence motifs involved in substrate recognition. YluC belongs to the site-2-protease (S2P) family of proteases involved in regulated intramembrane proteolysis (RIP), which plays a role in diverse regulatory phenomena from bacteria to man. The yluC mutant, and strains with N-terminal truncations of ftsL have a short cell phenotype, indicating that that FtsL is normally rate-limiting for division. Coexpression experiments of FtsL and YluC in Escherichia coli corroborated a model in which FtsL is directly cleaved by the membrane metalloprotease. The results shed new light on the regulation of cell division in B. subtilis and identify a novel class of targets for RIP.

Sel1-like repeat proteins in signal transduction

Solenoid proteins, which are distinguished from general globular proteins by their modular architectures, are frequently involved in signal transduction pathways. Proteins from the tetratricopeptide repeat (TPR) and Sel1-like repeat (SLR) families share similar alpha-helical conformations but different consensus sequence lengths and superhelical topologies. Both families are characterized by low sequence similarity levels, rendering the identification of functional homologous difficult. Therefore current knowledge of the molecular and cellular functions of the SLR proteins Sel1, Hrd3, Chs4, Nif1, PodJ, ExoR, AlgK, HcpA, Hsp12, EnhC, LpnE, MotX, and MerG has been reviewed. Although SLR proteins possess different cellular functions they all seem to serve as adaptor proteins for the assembly of macromolecular complexes. Sel1, Hrd3, Hsp12 and LpnE are activated under cellular stress. The eukaryotic Sel1 and Hrd3 proteins are involved in the ER-associated protein degradation, whereas the bacterial LpnE, EnhC, HcpA, ExoR, and AlgK proteins mediate the interactions between bacterial and eukaryotic host cells. LpnE and EnhC are responsible for the entry of L. pneumophila into epithelial cells and macrophages. ExoR from the symbiotic microorganism S. melioti and AlgK from the pathogen P. aeruginosa regulate exopolysaccaride synthesis. Nif1 and Chs4 from yeast are responsible for the regulation of mitosis and septum formation during cell division, respectively, and PodJ guides the cellular differentiation during the cell cycle of the bacterium C. crescentus. Taken together the SLR motif establishes a link between signal transduction pathways from eukaryotes and bacteria. The SLR motif is so far absent from archaea. Therefore the SLR could have developed in the last common ancestor between eukaryotes and bacteria.

Evidence for a novel protease governing regulated intramembrane proteolysis and resistance to antimicrobial peptides in Bacillus subtilis

Evidence is presented that the activation of the RNA polymerase sigma factor sigma(W) in Bacillus subtilis by regulated intramembrane proteolysis is governed by a novel, membrane-embedded protease. The sigma(W) factor is activated by proteolytic destruction of the membrane-bound anti-sigma(W) factor RsiW in response to antimicrobial peptides and other agents that damage the cell envelope. RsiW is destroyed by successive proteolytic events known as Site-1 and Site-2 cleavage. Site-2 cleavage is mediated by a member of the SpoIVFB-S2P family of intramembrane-acting metalloproteases, but the protease responsible for Site-1 cleavage was unknown. We have identified a previously uncharacterized, multipass membrane protein called PrsW (annotated YpdC) that is both necessary and sufficient (when artificially produced in an unrelated host bacterium) for Site-1 cleavage of RsiW. PrsW is a member of a widespread family of membrane proteins that includes at least one previously known protease. We identify residues important for proteolysis and a cluster of acidic residues involved in sensing antimicrobial peptides and cell envelope stress.