Source: http://www.antievolution.org/cgi-bin/ikonboard/ikonboard.cgi?s=5ba4cd0b75986bf1;act=ST;f=12;t=765;st=30
Timestamp: 2019-04-21 17:04:57+00:00

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Flagellar filaments, which act as propellers, consist of self-assembling protein subunits (flagellin) arranged in a helix and forming a hollow tube (reviewed in reference 135). Subunits move down the hollow core and are polymerized at the tip of the flagellum. The V. parahaemolyticus polar organelle is a complex flagellum. There are six polar flagellin genes, organized in two loci (86, 122). The flagellins are similar to each other: FlaB and FlaA are 78% identical, FlaB and FlaC are 68% identical, FlaB and FlaD are 99% identical, FlaB and FlaF are 69% identical, and FlaB and FlaE are 50% identical. Despite the great protein similarity of FlaB and FlaD, the flagellins migrate differently on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, suggesting the possibility of posttranslational modification, e.g., glycosylation, which has been observed for flagellins of many bacteria including some spirochetes, Campylobacter species, and P. aeruginosa (27, 56, 171, 196), or phosphorylation, which has been detected for flagellins of P. aeruginosa (85). Analysis of the protein composition of purified flagella from wild-type strains and strains with mutations in flagellin genes suggests that all of the flagellins can be incorporated into the organelle and that FlaA, FlaB, and FlaD are the major subunits (121, 122; L. McCarter, unpublished data). Nothing is known with respect to their spatial arrangement in the flagellum. Loss of function of a single flagellin gene has little or no effect on swimming motility or flagellar structure (waveform or length). Thus, none of the six flagellin genes is essential for filament formation. Deletion of the flaFBA or the flaCD genes also has little effect on motility, but the deletion of both loci (flaFBA flaCD) completely abolishes motility (122).
Why are there six flagellin genes? It is not clear why the organism possesses such an extraordinary number of flagellins. The similarity of the gene products and the dispensability of the genes suggest that there are no special structural requirements, although the filament structure and function could be more complex and adapted to specific circumstances than our laboratory tests can reveal. Bacteria are known to modulate the antigenicity of their flagellar filaments by expression of different flagellin genes or by recombination and rearrangement of flagellin genes (reviewed in reference 190). Therefore, the capacity for immune system evasion in a host organism might account for some of the diversity. The multiplicity of flagellin genes suggests a significant reservoir for antigenic or phase variation. Although the sheath covers the filament and might be thought to provide a disguise, electron microscopy suggests that it may be fragile (2, 164). Thus, the sheath may not protect the filament against the immune response of a host. In some respects the endoflagella of the spirochetes are similar, for these flagella can also be viewed as being sheathed, polar organelles (reviewed in reference 103). The spirochete flagella are normally found in the periplasm, between the outer membrane and the cell cylinder and attached near each cell pole. Purification of periplasmic flagella demonstrated that these filaments are also complex, with generally two to four different flagellin proteins, encoded by distinct genes, as well as containing an accessory, nonflagellin protein.
The flagellum is sheathed by an apparent extension of the cell membrane (2). The mechanism of how a sheathed flagellum rotates has not been elucidated. Potentially, the flagellar filament could rotate within the sheath or the two could rotate as a unit (50). Little is known about the composition, formation, or function of flagellar sheaths, which are found in many bacteria, including marine Vibrio species, V. cholerae, B. bacteriovorus, and Helicobacter pylori (reviewed in reference 164). Evidence from these organisms suggests that the sheath contains both lipopolysaccharide and proteins and that it may exist as a stable membrane domain distinct from the outer membrane (42, 51, 58, 69, 144). The lipid content of the sheath of B. bacteriovorus is distinct from that of the outer membrane, and the sheath appears to be a highly fluid, symmetric bilayer (179). How the sheath is formed remains essentially uninvestigated. It has been postulated that the sheath forms concomitantly with the elongation of the flagellar filament. However, it is provocative to note that "tubules" or structures that appear to be empty sheaths lacking filament have been observed, which suggests the interesting possibility of uncoupling of the flagellar core and the sheath assembly (2). One of three major sheath proteins of V. alginolyticus has been characterized. Genetic and biochemical evidence suggests that it is a lipoprotein (52). Another flagellar sheath protein that is a lipoprotein is HpaA of H. pylori (76, 144). There is some controversy about the role and cellular location of HpaA. Although HpaA has also been reported to be a cell surface adhesin (45), other groups have localized HpaA to the cytoplasm (144) or the flagellar sheath (76), and no adherence defect for hpaA mutants to eukaryotic cell lines has been demonstrated (76, 144). Experiments with V. anguillarum suggest the sheath is a virulence organelle. Mutants of V. anguillarum that lack a major flagellar sheath antigen are avirulent, even though the initial stages of infection are unaffected (137). Biochemical analysis indicates that this particular sheath antigen is lipopolysaccharide. Thus, the sheath may be important for specific interactions with the environment.
And, the presence of the sheath appears to make the loss of the distal cap (HAP2, here) non-fatal to flagellar function.
The sheath seems to provide some variation to the pathway of flagellar assembly. The hook-basal-body complex forms a channel through which proteins can be exported. In fact, not only are structural elements of the flagellum exported through this channel, but also regulatory molecules, e.g., the flagellar anti- factor FlgM, are secreted (70, 99, 139). For bacteria with unsheathed flagella, such as E. coli, mutants with defects in genes encoding three hook-associated proteins (HAPs) are nonmotile and secrete unpolymerized flagellin subunits (66). HAP1 and HAP3 are the connector proteins that join the filament to the hook. Without the ability to adapt flagellin subunits to the hook, the flagellins are secreted. HAP2 is also called the distal capping protein because its role is one of a cap or plug. Without this cap, flagellins are also secreted. Since purified flagellin subunits can assemble in vitro (9) and since an S. enterica serovar Typhimurium mutant lacking HAP2 can polymerize filaments if the concentration of flagellin in the external medium is high (67), the role of HAP2 has been viewed as capping the flagellar tip to retard subunit secretion sufficiently to increase the local concentration of flagellin and promote self-assembly. Recent work in Salmonella, analyzing cap-filament interactions by cryoelectron microscopy, suggests a model for the cap as being a flat, disklike pentameric structure that acts as a processive chaperone, preventing the loss of flagellin monomers and actively catalyzing folding and insertion into the filament (200).
HAP mutants of V. parahaemolyticus display different phenotypes (122). The most striking difference is in the phenotypes of mutants with defects in the gene encoding HAP2. These mutants are competent for filament assembly and motile. Figure 5 compares the flagella of the wild type and mutants with defects in the gene that encodes HAP2, and the flagella seem mostly indistinguishable. This suggests that in the absence of HAP2 but in the presence of the flagellar sheath, the local concentrations of the flagellin monomers remain high enough to allow polymerization of subunits. HAP1 and HAP3 mutants of V. parahaemolyticus are nonmotile and nonflagellated; however, they produce detached, severely truncated filaments encased in a membrane (122). The sheath seems to act to retain flagellin monomers and allow subunit assembly. The polymerized flagellins cannot be connected to the hook and bleb off as abortive filaments surrounded by a membrane vesicle. Similar filamentless mutants that produce flagellin-containing membrane vesicles were isolated in V. alginolyticus, although the genetic lesions in these strains were not determined (136). Thus, the sheath itself appears to be able to substitute for the cap.
The complement of chemotaxis (che) genes and their organization are different from those in E. coli. One of these genes is cheV, which encodes a hybrid CheY/CheW. CheV has been found in a number of organisms, including Bacillus species, Campylobacter jejuni, H. pylori, P. aeruginosa, and V. cholerae. In B. subtilis, genetic analysis suggests that CheV and CheW are functionally redundant (155). Three unusual ORFs occur within the che gene cluster of region 2. ORF1 encodes a protein that resembles Soj of B. subtilis and other ATPase proteins involved in chromosome partitioning (152, 161). The other ORFs encode potential polypeptides that do not resemble proteins of known function. It seems curious that a Soj-like protein exists within a flagellar/chemotaxis operon, and this particular arrangement is conserved in other bacteria, e.g., P. aeruginosa, P. putida, and V. cholerae. Perhaps these novel ORFs will prove key for understanding the linkage between cell division and flagellation or development. It should be remarked that many of the V. parahaemolyticus che genes located in the flagellar clusters (Table 2) were discovered by mutant analysis; i.e., these genes produce defects in chemotaxis when mutated. The V. cholerae genome, as well as those of Pseudomonas species, indicates additional complexity with respect to a multiplicity of potential che genes.
MotX and MotY proteins are the unique components of sodium-type flagellar systems, and their specific roles are not known. Loss of function of either motX or motY produces a paralyzed mutant completely defective for swimming but competent for flagellar assembly (123, 124, 141). Both proteins possess single membrane-spanning domains. The C terminus of MotY contains an extended domain that shows striking homology to a number of outer membrane proteins know to interact with peptidoglycan, e.g., OmpA and peptidoglycan-associated lipoproteins (124). The simplest hypothesis for the role of MotY is that the polar flagellar motor possesses two elements for anchoring the force generator. Perhaps extremely precise alignment of the stator with the rotor is required for a motor that spins as fast as 100,000 rpm. The role of MotX remains mysterious. It is known that MotX recruits MotY to the membrane when the proteins are coexpressed in E. coli (123). Furthermore, overexpression of MotX is lethal to E. coli in proportion to the external Na+ concentration, and lethality can be reversed by the presence of the sodium channel blocker amiloride. This suggests that the proteins may somehow participate in or modulate Na+ translocation. For example, MotX could act to modify or specify ion channel activity. Thus, it may be that all four proteins comprise and specify the sodium-type torque-generating unit. However, there is no existing evidence that places MotX and MotY in the physical context of MotA and MotB. It is possible that MotX and MotY may play a more distinct role in the generation of sodium-driven motility; e.g., they could participate in some other aspect of the sodium cycle.
The genomes of V. cholerae and P aeruginosa are tantalizing with respect to chemotaxis, for they suggest additional complexity over that of E. coli due to a multiplicity of che-like genes. For example, five cheY-like genes can be identified in V. cholerae.
Flagella, which act as semirigid helical propellers, provide bacteria with a highly efficient means of locomotion. For example, many Vibrio and Pseudomonas species swim in liquid environments at speeds as fast as 60 �m/s (10, 11, 64, 189). The propellers are powered by reversible rotary motors embedded in the cell membrane, which can turn the flagellum at rates as high as 1,700 revolutions per s (rps) (115). Energy for rotation of the motor is derived from either the sodium or proton membrane potential (72, 117). The number and arrangement of the propellers can vary, but the mode of insertion is of two major types, i.e., polar or peritrichous. Flagella play other roles in addition to swimming in liquid (reviewed in reference 132). They can enable bacteria to move over and colonize surfaces, a process called swarming (63). They also participate in adhesion. Attachment of bacteria to surfaces is often first mediated by contact of the flagellum with the surface (127). As propulsive organelles, flagella seems to aid in overcoming negative electrostatic interactions and thus are believed to play a key role in the initial steps of adsorption of bacteria to surfaces, biofilm formation, and invasion of hosts (30, 39, 154). Studies using Vibrio alginolyticus have demonstrated that attachment to glass is directly proportional to swimming speed (90). Other studies have shown that by disabling the flagellar motor of the fish pathogen V. anguillarum, invasion into the fish host is severely reduced (142). In addition, some flagella are sheathed by a membrane that appears to be an extension of the outer cell membrane. The composition of this sheath (specifically, lipopolysaccharide and protein) may allow additional specific interactions between the bacterium and a surface (77, 163, 164).
F. A. Janssens Laboratory of Genetics, Katholieke Universiteit Leuven, Heverlee, Belgium.
Many bacterial species are motile by means of flagella. The structure and implantation of flagella seems related to the specific environments the cells live in. In some cases, the bacteria even adapt their flagellation pattern in response to the environmental conditions they encounter. Swarming cell differentiation is a remarkable example of this phenomenon. Flagella seem to have more functions than providing motility alone. For many pathogenic species, studies have been performed on the contribution of flagella to the virulence, but the result is not clear in all cases. Flagella are generally accepted as being important virulence factors, and expression and repression of flagellation and virulence have in several cases been shown to be linked. Providing motility is always an important feature of flagella of pathogenic bacteria, but adhesive and other properties also have been attributed to these flagella. In nonpathogenic bacterial colonization, flagella are important locomotive and adhesive organelles as well. In several cases where competition between several bacterial species exists, motility by means of flagella is shown to provide a specific advantage for a bacterium. This review gives an overview of studies that have been performed on the significance of flagellation in a wide variety of processes where flagellated bacteria are involved.
133. Motaleb, M. A., L. Corum, J. L. Bono, A. F. Elias, P. Rosa, D. S. Samuels, and N. W. Charon. 2000. Borrelia burgdorferi periplasmic flagella have both skeletal and motility functions. Proc. Natl. Acad. Sci. USA 97:10899-10904 Abstract/Free Full Text.
Department of Botany, University of Bristol, England.
The gas vesicle is a hollow structure made of protein. It usually has the form of a cylindrical tube closed by conical end caps. Gas vesicles occur in five phyla of the Bacteria and two groups of the Archaea, but they are mostly restricted to planktonic microorganisms, in which they provide buoyancy. By regulating their relative gas vesicle content aquatic microbes are able to perform vertical migrations. In slowly growing organisms such movements are made more efficiently than by swimming with flagella. The gas vesicle is impermeable to liquid water, but it is highly permeable to gases and is normally filled with air. It is a rigid structure of low compressibility, but it collapses flat under a certain critical pressure and buoyancy is then lost. Gas vesicles in different organisms vary in width, from 45 to > 200 nm; in accordance with engineering principles the narrower ones are stronger (have higher critical pressures) than wide ones, but they contain less gas space per wall volume and are therefore less efficient at providing buoyancy. A survey of gas-vacuolate cyanobacteria reveals that there has been natural selection for gas vesicles of the maximum width permitted by the pressure encountered in the natural environment, which is mainly determined by cell turgor pressure and water depth. Gas vesicle width is genetically determined, perhaps through the amino acid sequence of one of the constituent proteins. Up to 14 genes have been implicated in gas vesicle production, but so far the products of only two have been shown to be present in the gas vesicle: GvpA makes the ribs that form the structure, and GvpC binds to the outside of the ribs and stiffens the structure against collapse. The evolution of the gas vesicle is discussed in relation to the homologies of these proteins.

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