Joined: May 2002
A coupla recent flag-related articles, my summaries of evo-interesting bits in .
Proc Natl Acad Sci U S A 2003 Mar 18;100(6):3027-30
Type III secretion systems and bacterial flagella: Insights into their function from structural similarities.
Blocker A, Komoriya K, Aizawa S.
Sir W. Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, United Kingdom; and Department of Biosciences, Teikyo University, Utsunomiya 320-8551, Japan.
Type III secretion systems and bacterial flagella are broadly compared at the level of their genetic structure, morphology, regulation, and function, integrating structural information, to provide an overview of how they might function at a molecular level.
[More homologies to flagellar proteins than the standard ~9 or so: ]
Old and New Sequence Homologies.
TTS apparatuses are encoded by 25 genes (4), nearly all essential for function. About 10 TTSS proteins are similar in sequence or membrane topology to cytoplasmic or inner membrane proteins of flagellar hook-basal bodies (HBBs; refs. 5 and 6). Others show no significant sequence homology. However, they show "functional conservation" because when knocked out, they lead to similar phenotypes in assembly or function of the apparatuses. By matching biochemical characteristics and biological information about each protein (see Supporting Text, which is published as supporting information on the PNAS web site, www.pnas.org), we propose the functional homologs shown in Fig. 1.
A part of the TTS machinery, the "needle complex" (NC) resembles HBBs (6, 7). NCs comprise a 10 × 60-nm external needle inserted within a 30-nm (in diameter) cylinder traversing both bacterial membranes and the peptidoglycan. The Shigella secreton has an additional "bulb," 45 × 25 nm, on the cytoplasmic side of the inner membrane, similar to the flagellar C-ring (refs. 8 and 9; Fig. 1). NCs are traversed by a 2- to 3-nm channel (10), which exists also within the entire bacterial flagellum (11). Flagellin may transit partially unfolded (12) through this channel to its tip, where it refolds and inserts into the growing filament (13, 14). Effectors from plant pathogen TTSSs are also secreted from the distal tip of their TTS machineries (15). During assembly of flagella in vivo, a cap is added before each transition to a new part of the flagellum so new subunits, which would otherwise diffuse away, can be inserted directly under the cap (16). NC components, including the needle component MxiH/PrgI, have been identified (17, 18). No cap has been identified in any TTSS. Morphological divergence between TTSSs is discussed in Supporting Text.
What are the energizers of posttranslational and cotranslational secretion?
The flagellar ATPase FliI is required for export of all flagellar proteins except the outer membrane components (5). Without it only the inner membrane and cytoplasmic components are assembled. Mutants in homologous TTSS ATPases display analogous phenotypes (65). Does cotranslational secretion occur by docking of the ribosome to the cytoplasmic part of the TTS machine like cotranslational export across the membrane of the endoplasmic reticulum? An empty flagellar C-ring could only accommodate two ribosomes, the protein channels of which could not directly dock to the HBB without a gap being left. Therefore, cotranslational secretion is probably also driven by the ATPase and hence indirect.
How is energy transduced by the export motor during secretion?
The ATPases interact with cytoplasmic components of TTSSs or flagella but the function(s) of these interactions are mostly unidentified (66, 67). The biological cycle of the enzyme is unknown and its localization is debated [cytosolic or membrane-bound (69)?]. How might these ATPases catalyze processive protein export? Spa47 (the Shigella FliI homolog) shares 33% amino acid identity with the -subunit of F1-ATPase. Proteins with >30% sequence identity have a high probability of sharing similar structures (69). Active F1-ATPase is a heterohexamer consisting of alternating - and -subunits with a -subunit inserted in a central channel where it rotates during the catalytic cycle (70). No equivalent of the -subunit of F1-ATPases is found within flagellar or TTSS-encoding operons, so we assume that the type III export motor is a homohexamer. When modeled on the F1 structure, Spa47 fits at the inner membrane base of our NC structure (Fig. 3). It would contain a central channel aligned with the one found within the NC and of similar diameter to it, through which the proteins could be secreted (see Supporting Text).
[It would be very interesting if the FliI ATPase really was a homohexamer like the F1 subunits in the F1F0 ATPase. Not determinative of anything I suppose but it would make Rizzotti's model a bit more likely]
Our understanding of TTSSs was applied to obtain an MHC class I response against a heterologous translocated protein (75). TTSSs are targets for new antimicrobial drugs. Work on type IV secretion systems in other Gram-negative pathogens shows that they, too, can perform host cell contact-mediated protein translocation (76). Type IV secretion apparatuses resemble bacterial conjugation systems, which function differently from TTSSs. The sec-dependent secretion pathway of Gram-positive bacteria also seems capable of polarized protein translocation into host cells (77). These may be examples of convergent evolution.
Mol Cell Probes 2003 Feb;17(1):25-32
Detection of type III secretion genes as a general indicator of bacterial virulence.
Stuber K, Frey J, Burnens AP, Kuhnert P.
Institute of Veterinary Bacteriology, University of Bern, Langgassstrasse 122, CH-3012, Bern, Switzerland
Type III secretion systems of Gram-negative bacteria are specific export machineries for virulence factors which allow their translocation to eukaryotic cells. Since they correlate with bacterial pathogenicity, their presence is used as a general indicator of bacterial virulence. By comparing the genetic relationship of the major type III secretion systems we found the family of genes encoding the inner-membrane channel proteins represented by the Yersinia enterocolitica lcrD (synonym yscV) and its homologous genes from other species an ideal component for establishing a general detection approach for type III secretion systems. Based on the genes of the lcrD family we developed gene probes for Gram-negative human, animal and plant pathogens. The probes comprise lcrD from Y. enterocolitica, sepA from enteropathogenic Escherichia coli, invA from Salmonella typhimurium, mxiA from Shigella sonnei, as well as hrcV from Erwinia amylovora. In addition we included as a control probe the flhA gene from E. coli K-12 to validate our approach. FlhA is part of the flagellar export apparatus which shows a high degree of similarity with type III secretions systems, but is not involved in pathogenicity. The probes were evaluated by screening a series of pathogenic as well as non-pathogenic bacteria. The probes detected type III secretion in pathogens where such systems were either known or were expected to be present, whereas no positive hybridization signals could be found in non-pathogenic Gram-negative bacteria. Gram-positive bacteria were devoid of known type III secretion systems. No interference due to the genetic similarity between the type III secretion system and the flagellar export apparatus was observed. However, potential type III secretion systems could be detected in bacteria where no such systems have been described yet. The presented approach provides therefore a useful tool for the assessment of the virulence potential of bacterial isolates of human, animal and plant origin. Moreover, it is a powerful means for a first safety assessment of poorly characterized strains intended to be used in biotechnological applications.
[short version: no evidence of basal Type III secretion systems, but then this technique would probably only detect T3SS within the flagellum-derived-virulence-system "clade" anyhow...
Note that e.g. gram-positive bacteria have regular flagella, so when the authors say "Gram-positive bacteria were devoid of known type III secretion systems" they are not including flagella.]
Trends Cell Biol 2003 Mar;13(3):114-21
Rotary protein motors.
Oster G, Wang H.
Depts Molecular and Cellular Biology and ESPM, College of Natural Resources, University of California, 94720, Berkeley, CA, USA
THREE PROTEIN MOTORS HAVE BEEN UNAMBIGUOUSLY IDENTIFIED AS ROTARY ENGINES: the bacterial flagellar motor and the two motors that constitute ATP synthase (F(0)F(1) ATPase). Of these, the bacterial flagellar motor and F(0) motors derive their energy from a transmembrane ion-motive force, whereas the F(1) motor is driven by ATP hydrolysis. Here, we review the current understanding of how these protein motors convert their energy supply into a rotary torque.
[*two* motors in the F1F0 ATPase? This is a different interpretation. Anyhoo, no full-text access to this one for me.
J Mol Biol 2003 Mar 21;327(2):453-63
Ion-coupling Determinants of Na(+)-driven and H(+)-driven Flagellar Motors.
Asai Y, Yakushi T, Kawagishi I, Homma M.
Division of Biological Science, Graduate School of Science, Nagoya University, Chikusa-Ku, 464-8602, Nagoya, Japan
The bacterial flagellar motor is a tiny molecular machine that uses a transmembrane flux of H(+) or Na(+) ions to drive flagellar rotation. In proton-driven motors, the membrane proteins MotA and MotB interact via their transmembrane regions to form a proton channel. The sodium-driven motors that power the polar flagellum of Vibrio species contain homologs of MotA and MotB, called PomA and PomB. They require the unique proteins MotX and MotY. In this study, we investigated how ion selectivity is determined in proton and sodium motors. We found that Escherichia coli MotA/B restore motility in DeltapomAB Vibrio alginolyticus. Most hypermotile segregants isolated from this weakly motile strain contain mutations in motB. We constructed proteins in which segments of MotB were fused to complementary portions of PomB. A chimera joining the N terminus of PomB to the periplasmic C terminus of MotB (PotB7(E)) functioned with PomA as the stator of a sodium motor, with or without MotX/Y. This stator (PomA/PotB7(E)) supported sodium-driven motility in motA or motB E.coli cells, and the swimming speed was even higher than with the original stator of E.coli MotA/B. We conclude that the cytoplasmic and transmembrane domains of PomA/B are sufficient for sodium-driven motility. However, MotA expressed with a B subunit containing the N terminus of MotB fused to the periplasmic domain of PomB (MomB7(E)) supported sodium-driven motility in a MotX/Y-dependent fashion. Thus, although the periplasmic domain of PomB is not necessary for sodium-driven motility in a PomA/B motor, it can convert a MotA/B proton motor into a sodium motor.
[interesting bit of mixing and matching]