The unusual extended signal peptide region of the type V secretion system is phylogenetically restricted

Abstract

The plasmid encoded toxin, Pet, is a prototypical member of the serine protease autotransporters of the Enterobacteriaceae. In addition to the passenger and beta-domains typical of autotransporters, in silico predictions indicate that Pet possesses an unusually long N-terminal signal sequence. The signal sequence can be divided into five regions termed N1 (charged), H1 (hydrophobic), N2, H2 and C (cleavage site) domains. The N1 and H1 regions, which we have termed the extended signal peptide region, demonstrate remarkable conservation. In contrast, the N2, H2 and C regions demonstrate significant variability and are reminiscent of typical Sec-dependent signal sequences. Despite several investigations, the function of the extended signal peptide region remains obscure and surprisingly it has not been proven that the extended signal peptide region is actually synthesized as part of the signal sequence. Here, we demonstrate that the extended signal peptide region is present only in Gram-negative bacterial proteins originating from the classes Beta- and Gammaproteobacteria, and more particularly only in proteins secreted via the Type V secretion pathway: autotransporters, TpsA exoproteins of the two-partner system and trimeric autotransporters. In vitro approaches demonstrate that the DNA region encoding the extended signal peptide region is transcribed and translated.

Trimeric Autotransporters ofHaemophilus parasuis: Generation of an Extensive Passenger Domain Repertoire Specific for Pathogenic Strains

“…Furthermore, a closer analysis of the first VtaA amino acids indicated conservation with an extended signal peptide characteristic of many gamma-and betaproteobacterium autotransporters. Indeed, the motif MNKIF/Y was fully conserved, and the first 72 amino acids of VtaA contained alternating charged small hydrophilic regions with more extended hydrophobic regions (13).…”

Shigella

“…This N-terminal secretion signal is atypically long in some cases, such as in members of the Ser protease autotransporters of the Enterobacteriaceae (SPATE) family. 112 The C-terminal domain of an autotransporter is the actual translocation unit needed to bypass the outer membrane. This domain will form a β-barrel structure in the outer membrane to assist the translocation of the effector or passenger domain of the autotransporter to the extracellular milieu (see Fig.…”

The Extended Signal Peptide of the Trimeric Autotransporter EmaA of Aggregatibacter actinomycetemcomitans Modulates Secretion

“…1A and B) (11,47). Some long signal peptides may be composed of two divergent NH regions followed by a C region, denoted N1H1N2H2C, with little sequence similarity between the individual N and H regions (11,17). The role of the N-terminal extension remains to be elucidated.…”

“…Conversely, the amino-terminal amino acids contain a unique, highly conserved sequence motif, referred to as the N-terminal extension or extended signal peptide region (ESPR) (Fig. 1A and B) (11,47). Some long signal peptides may be composed of two divergent NH regions followed by a C region, denoted N1H1N2H2C, with little sequence similarity between the individual N and H regions (11,17).…”