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.