Type V secretion: mechanism(s) of autotransport through the bacterial outer membrane

Abstract

Autotransport in Gram-negative bacteria denotes the ability of surface-localized proteins to cross the outer membrane (OM) autonomously. Autotransporters perform this task with the help of a β-barrel transmembrane domain localized in the OM. Different classes of autotransporters have been investigated in detail in recent years; classical monomeric but also trimeric autotransporters comprise many important bacterial virulence factors. So do the two-partner secretion systems, which are a special case as the transported protein resides on a different polypeptide chain than the transporter. Despite the great interest in these proteins, the exact mechanism of the transport process remains elusive. Moreover, different periplasmic and OM factors have been identified that play a role in the translocation, making the term ‘autotransport’ debatable. In this review, we compile the wealth of details known on the mechanism of single autotransporters from different classes and organisms, and put them into a bigger perspective. We also discuss recently discovered or rediscovered classes of autotransporters.

Complete fiber structures of complex trimeric autotransporter adhesins conserved in enterobacteria

“…Adhesion to host cells is a key event during the onset of infection; the mediators of this process, called adhesins, are a heterogeneous group of bacterial surface proteins, which vary in architecture, domain content, and mode of binding. One distinct class of adhesins are the trimeric autotransporter adhesins (TAAs) (2,3), also referred to as type Vc secretion systems (4). TAAs are important virulence factors of many well-studied pathogens: Examples include YadA of Yersinia enterocolitica, a species causing enteritis, mesenteric lymphadenitis, and reactive arthritis (5); NadA of Neisseria meningitidis (6), an agent of meningitis and sepsis; BadA of Bartonella henselae (7), which is the agent of cat scratch disease; UspA1 and A2 of Moraxella catarrhalis (8), a prominent species in respiratory tract infections, and Hia of Haemophilus influenza (9), an organism causing meningitis and respiratory tract infections.…”

A Review of Secretion Systems in Pathogenic and Non-Pathogenic Bacteria

“…A large group of "Other T4SSs"include other types, which consist of G1 T4SSs coded by a number of genomic islands, such as clc, pKLC102 and PAPI from Pseudomonas, SPI-7 from Salmonella and also a group of integrated conjugation elements (ICE) from Haemophilus 34 6,37 . VirB4, VirB11 and VirD4 ATPase provide required energy for Pilus biogenesis and transporting sub-material through T4SS channel.VirD4 makes conjugator protein double to emphasize on its imminent impact in recognizing and using the substrate in the T4SS secretion channel 6,32,37 . A number of toxic genes resistant to antibiotics and dependent on catabolism intensify virulence and general bacterial compliance through transporting materials by T4SS.…”

“…Additionally, to transport and fix nitrogen in plants, Mesorhizobium loti and Rhizobium leguminosarum are also required. T6SS coding genes are also found in some nonsymbionts such as Myxococcus xanthus, Dechloromonas aromatica and Rhodopirellula baltica, and it is possible to form biofilm to comply with condition 37 .…”

“…These clusters may show system specific substrates and protective or regulatory proteins. Various genomicanalysesindicate that T6SSs are almost present in a quarter of the bacterial genomes of sequencing and have extended among proteobacteria (except epsilon proteobacteria) and at least one third of the bacteria have several T6SSs, and this amount is more than 6 times in Yersinia pestis, Burkholderiapseudomallei [37][38] .These analyses show that T6SSs can be classified to 4 families or phylogenetic groups A, B, C, D 1,6 that are corresponding to the I, II,V,IV 37 , although, a complete structure is not provided for T6SS subcompound like available structures for T3SS and T4SS 38 . It has recently been reported that the antibacterial T6SSs have been invented and effectively used to fight bacterial cells 38,39 .…”

Haemophilus influenzae surface fibril (Hsf) is a unique twisted hairpin-like trimeric autotransporter

“…Unlike the type I-IV secretory systems that involve multiple proteins, autotransporters are composed of a single protein consisting of an N-terminal signal peptide for secretion, followed by a passenger domain, and a C-terminal translocator (membrane anchoring) domain Leo et al, 2012). These autotransporters are multifunctional proteins ranging from monomeric to multimeric arrangements (Meng et al, 2011).…”