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.

Gram-negative bacteria commonly interact with animal and plant hosts using type III secretion systems (TTSSs) for translocation of proteins into eukaryotic cells during infection. 10 of the 25 TTSS-encoding genes are homologous to components of the bacterial flagellar basal body, which the TTSS needle complex morphologically resembles. This indicates a common ancestry, although no TTSS sequence homologues for the genes encoding the flagellum are found. We here present an ϳ16-Å structure of the central component, the needle, of the TTSS. Although the needle subunit is significantly smaller and shares no sequence homology with the flagellar hook and filament, it shares a common helical architecture (ϳ5.6 subunits/turn, 24-Å helical pitch). This common architecture implies that there will be further mechanistic analogies in the functioning of these two bacterial systems.

SummaryWe analysed all major proteins secreted into culture media from Salmonella typhimurium. Proteins in culture supernatants were collected by trichloroacetic acid precipitation, separated in SDS±polyacrylamide gels and analysed by amino acid sequencing. Wildtype strain SJW1103 cells typically gave rise to nine bands in SDS gels: 89, 67, 58, 52, 50, 42, 40, 35 and (sometimes) 28 kDa. A search of the sequences in the available databases revealed that they were either agellar proteins or virulence factors. Six of them were¯agella speci®c: FlgK or HAP1 (58 kDa), FliC or agellin (52 kDa), FliD or HAP2 (50 kDa), FlgE or hook protein (42 kDa), FlgL or HAP3 (35 kDa) and FlgD or hook-cap protein (28 kDa). The other four bands were speci®c for virulence factors: SipA (89 kDa), SipB (67 kDa), SipC (42 kDa) and InvJ (40 kDa). The 42 kDa band was a mixture of FlgE and SipC. We also analysed secreted proteins from more than 30¯agellar mutants, and they were categorized into four groups according to their band patterns: wild type, mot type, polyhook type and master gene type. Virulence factors were constantly secreted at a higher level in all¯agellar mutants except a Dmot (motAB deletion) mutant, in which the amounts were greatly reduced. A new morphological pathway of¯agellar biogenesis including protein secretion is presented.

Length determination in biology generally uses molecular rulers. The hook, a part of the flagellum of motile bacteria, has an invariant length. Here, we examined hook length and found that it was determined not by molecular rulers but probably by the amount of subunit protein secreted by the flagellar export apparatus. The export apparatus shares common features with the type III virulence-factor secretion machinery and thus may be used more widely in length determination of structures other than flagella.

While they are functionally similar, archaeal flagella have characteristics not typically seen in bacterial flagella (37). Archaeal flagellins are synthesized with a leader peptide (3, 18), which is cleaved prior to the incorporation of the flagellin into the filament, similar to the processing of type IV pilins before incorporation into the pilus (34). A Methanococcus maripaludis protein (FlaK) possessing preflagellin peptidase activity has recently been reported (2). Archaeal flagellins show sequence similarity to type IV pilins at the N termini of the mature proteins (8) and do not demonstrate homology to bacterial flagellins. Archaeal flagella are thinner in diameter (10 to 13 nm [4,16,33]) than bacterial flagella (20 nm [17]) and are always composed of multiple flagellins, which are often glycosylated (6,20,24). Additionally, a search of completely sequenced archaeal genomes failed to identify genes homologous to any genes coding for structural proteins involved in bacterial flagellation (7). This includes, but is not limited to, genes encoding the hook, rod, or ring proteins. All of these characteristics suggest that the structural components of the archaeal flagella are composed of unique, archaeon-specific proteins, possibly fulfilling the same function as those present in bacterial flagella, and that the mode of assembly is likely distinct as well.Methanococcus voltae is a marine organism possessing more than 70 flagella on the cell surface. As is typical of archaeal flagella, M. voltae flagella are composed of multiple flagellins (18). There are four flagellin genes found within two transcriptional units in the M. voltae chromosome, with the first transcriptional unit containing a single flagellin gene, flaA. The second transcriptional unit includes the three remaining flagellin genes (flaB1, flaB2, and flaB3) and the downstream cotranscribed flagellar accessory genes flaCDEFGHIJ (18; N. A. Thomas and K. F. Jarrell, unpublished data). Purified flagella were shown to be composed of two major proteins, flagellins FlaB1 and FlaB2, with molecular masses corresponding to 33 and 31 kDa, respectively (18). Prior to the work presented in this study, the remaining two flagellins (FlaA and FlaB3) remained undetected.Within the flagellated archaea, little work has been done to address the universal presence of multiple flagellins. In Halobacterium salinarum, five flagellin genes are arranged in two different loci. Two flagellin genes (flgA1 and flgA2) are arranged in tandem at one locus, with the remaining three genes (flgB1, flgB2, and flgB3) clustered in another locus, and all five corresponding gene products have been identified within isolated flagella (9). The estimated lengths of the mRNAs indicate that the flagellin genes within each locus are cotranscribed, but the transcripts did not include the accessory genes as seen in M. voltae and other methanogens (10, 38). It was recently determined that the majority of the accessory genes observed in M. voltae are present in H. salinarum next to the flgB locus bu...

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