Homologues of the protein constituents of the Klebsiella pneumoniae (Klebsiella oxytoca) type II secreton (T2S), the Pseudomonas aeruginosa type IV pilus/fimbrium biogenesis machinery (T4P) and the Methanococcus voltae flagellum biogenesis machinery (Fla) have been identified. Known constituents of these systems include (1) a major prepilin (preflagellin), (2) several minor prepilins (preflagellins), (3) a prepilin (preflagellin) peptidase/methylase, (4) an ATPase, (5) a multispanning transmembrane (TM) protein, (6) an outer-membrane secretin (lacking in Fla) and (7) several functionally uncharacterized envelope proteins. Sequence and phylogenetic analyses led to the conclusion that, although many of the protein constituents are probably homologous, extensive sequence divergence during evolution clouds this homology so that a common ancestry can be established for all three types of systems for only two constituents, the ATPase and the TM protein. Sequence divergence of the individual T2S constituents has occurred at characteristic rates, apparently without shuffling of constituents between systems. The same is probably also true for the T4P and Fla systems. The family of ATPases is much larger than the family of TM proteins, and many ATPase homologues function in capacities unrelated to those considered here. Many phylogenetic clusters of the ATPases probably exhibit uniform function. Some of these have a corresponding TM protein homologue although others probably function without one. It is further shown that proteins that compose the different phylogenetic clusters in both the ATPase and the TM protein families exhibit unique structural characteristics that are of probable functional significance. The TM proteins are shown to have arisen by at least two dissimilar intragenic duplication events, one in the bacterial kingdom and one in the archaeal kingdom. The archaeal TM proteins are twice as large as the bacterial TM proteins, suggesting an oligomeric structure for the latter. OverviewThree related types of prokaryotic envelope protein complexes include (putative) prepilin proteins with highly similar hydrophobic N-terminal segments of approximately 20 amino acyl residues. These putative prepilins can assemble into filamentous structures which compose parts of (1) the type II secretion system (T2S), (2) the type IV piliation/fimbriation system (T4P) (both of Gramnegative bacteria) and (3) the flagellar system (Fla) of archaea. T2S, also called the type II secreton or the main terminal branch (MTB; TC #3.A.15) of the general secretory pathway (TC #3.A.5;Cao & Saier, 2003), represents the major pathway for exoprotein transport from the periplasm across the outer membrane in a wide variety of Gramnegative bacteria (Pugsley, 1993a). The type II secreton is composed of a core of around 12 proteins, some of which are not present in all type II secretons and others of which appear to be dispensable for secreton function Pugsley, 1993a;Sandkvist, 2001). In this review, secreton components will be referred to accordi...

The signal transduction ATPases with numerous domains (STAND) represent a newly recognized class of widespread, sophisticated ATPases that are related to the AAA+ proteins and that function as signaling hubs. These proteins control diverse biological processes in bacteria and eukaryotes, including gene expression, apoptosis, and innate immunity responses. They function as tightly regulated switches, with the off and on positions corresponding to a long-lived monomeric, ADP-bound form and a multimeric, ATP-bound form, respectively. Inducer binding to the sensor domain activates the protein by promoting ADP for ATP exchange, probably through removal of an intramolecular inhibitory interaction, whereas ATP hydrolysis turns off the protein. One key component of the switch is a three-domain module carrying the ATPase activity (nucleotide-binding oligomerization domain [NOD]). Analysis of the atomic structures of four crystallized nucleotide-bound NOD modules provides an unprecedented insight into the NOD conformational changes underlying the activation process.

Lipoproteins in Gram-negative Enterobacteriaceae carry three fatty acids on the N-terminal cysteine residue, two as a diacylglyceride and one through an Nlinkage following signal peptide cleavage. Most lipoproteins are anchored in the outer membrane, facing the periplasm, but some lipoproteins remain in the plasma membrane, depending on the amino acid at position ؉2, immediately after the fatty-acylated cysteine. In vitro, the last step in lipoprotein maturation, N-acylation of apolipoproteins by the plasma membrane apolipoprotein N-acyltransferase (Lnt), is necessary for efficient recognition of outer membrane lipoproteins by the Lol system, which transports them from the plasma to the outer membrane (Fukuda, A., Matsuyama, S.-I., Hara, T., Nakayama, J., Nagasawa, H., and Tokuda, H. (2002) J. Biol. Chem. 277, 43512-43518). To study the role of Lnt in vivo, we constructed a conditional lnt mutant of Escherichia coli. The apo-form of peptidoglycan-anchored major lipoprotein (Lpp) and two other outer membrane lipoproteins accumulated in the plasma membrane when lnt expression was reduced. We also found that Lnt is an essential protein in E. coli and that the lethality is partially because of the retention of apoLpp in the plasma membrane. Topology mapping of Lnt with ␤-galactosidase and alkaline phosphatase fusions indicated the presence of six membrane-spanning segments. The lnt gene in a mutant of Salmonella enterica displaying thermosensitive Lnt activity