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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...

Only one of the characterized components of the main terminal branch of the general secretory pathway (GSP) in Gram‐negative bacteria, GspD, is an integral outer membrane protein that could conceivably form a channel to permit protein transport across this membrane. PulD, a member of the GspD protein family required for pullulanase secretion by Klebsiella oxytoca, is shown here to form outer membrane‐associated complexes which are not readily dissociated by SDS treatment. The outer membrane association of PulD is absolutely dependent on another component of the GSP, the outer membrane‐anchored lipoprotein PulS. Furthermore, the absence of PulS resulted in limited proteolysis of PulD and caused induction of the so‐called phage shock response, as measured by increased expression of the pspA gene. We propose that PulS may be the first member of a new family of periplasmic chaperones that are specifically required for the insertion of a group of outer membrane proteins into this membrane. PulS is only the second component of the main terminal branch of the GSP for which a precise function can be proposed.