Structures and functions of autotransporter proteins in microbial pathogens

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The ability to control the timing and mode of host cell death plays a pivotal role in microbial infections. Many bacteria use toxins to kill host cells and evade immune responses. Such toxins are unknown in Mycobacterium tuberculosis. Virulent M. tuberculosis strains induce necrotic cell death in macrophages by an obscure molecular mechanism. Here we show that the M. tuberculosis protein Rv3903c (channel protein with necrosis-inducing toxin, CpnT) consists of an N-terminal channel domain that is used for uptake of nutrients across the outer membrane and a secreted toxic C-terminal domain. Infection experiments revealed that CpnT is required for survival and cytotoxicity of M. tuberculosis in macrophages. Furthermore, we demonstrate that the C-terminal domain of CpnT causes necrotic cell death in eukaryotic cells. Thus, CpnT has a dual function in uptake of nutrients and induction of host cell death by M. tuberculosis.transport | pore | secretion

Section: Discussionmentioning

confidence: 99%

An outer membrane channel protein of Mycobacterium tuberculosis with exotoxin activity

Danilchanka¹,

Sun²,

Pavlenok³

et al. 2014

115

136

“…Ag43 | virulence factor | structural biology | urinary tract infection A utotransporter (AT) proteins represent a large family of diverse secreted and outer-membrane proteins present in numerous Gram-negative bacteria, including all classes of Proteobacteria as well as numerous species of Chlamydiales and Fusobacteria (1)(2)(3)(4). They are a core component of a molecular mechanism for the delivery of cargo protein domains across the outer membrane, and are often associated with important biological functions such as adhesion, aggregation, toxicity, and biofilm formation.…”

mentioning

confidence: 99%

The antigen 43 structure reveals a molecular Velcro-like mechanism of autotransporter-mediated bacterial clumping

Heras

Totsika

Peters

et al. 2013

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190

Aggregation and biofilm formation are critical mechanisms for bacterial resistance to host immune factors and antibiotics. Autotransporter (AT) proteins, which represent the largest group of outer-membrane and secreted proteins in Gram-negative bacteria, contribute significantly to these phenotypes. Despite their abundance and role in bacterial pathogenesis, most AT proteins have not been structurally characterized, and there is a paucity of detailed information with regard to their mode of action. Here we report the structure-function relationships of Antigen 43 (Ag43a), a prototypic self-associating AT protein from uropathogenic Escherichia coli. The functional domain of Ag43a displays a twisted L-shaped β-helical structure firmly stabilized by a 3D hydrogenbonded scaffold. Notably, the distinctive Ag43a L shape facilitates self-association and cell aggregation. Combining all our data, we define a molecular "Velcro-like" mechanism of AT-mediated bacterial clumping, which can be tailored to fit different bacterial lifestyles such as the formation of biofilms.Ag43 | virulence factor | structural biology | urinary tract infection

“…1E). Therefore, it is unlikely that residues 1342-1515 form part of the nucleation-elongation region of Sca2, but may instead function as an autochaperone domain, which are found in this location in most if not all autotransporters (32). Despite the differences between Sca2 and other passenger domains, the helix-loop-helix repeat observed here for NRD, which we propose is also present in CRD, resembles that of other autotransporters in that it is repetitive and local, suggesting that, like β-helical passengers domains (24,33), Sca2 also folds in a sequential (or vectorial) manner.…”

mentioning

confidence: 99%

Rickettsia Sca2 has evolved formin-like activity through a different molecular mechanism

Madasu

Suarez

Kast

et al. 2013

Sca2 (surface cell antigen 2) is the only bacterial protein known to promote both actin filament nucleation and profilin-dependent elongation, mimicking eukaryotic formins to assemble actin comet tails for Rickettsia motility. We show that Sca2's functional mimicry of formins is achieved through a unique mechanism. Unlike formins, Sca2 is monomeric, but has N-and C-terminal repeat domains (NRD and CRD) that interact with each other for processive barbed-end elongation. The crystal structure of NRD reveals a previously undescribed fold, consisting of helix-loop-helix repeats arranged into an overall crescent shape. CRD is predicted to share this fold and might form together with NRD, a doughnut-shaped formin-like structure. In between NRD and CRD, proline-rich sequences mediate the incorporation of profilin-actin for elongation, and WASP-homology 2 (WH2) domains recruit actin monomers for nucleation. Sca2's α-helical fold is unusual among Gram-negative autotransporters, which overwhelmingly fold as β-solenoids. Rickettsia has therefore "rediscovered" formin-like actin nucleation and elongation.passenger domain | translocator domain | spotted fever M any bacterial pathogens use the actin cytoskeleton of host eukaryotic cells for invasion and motility (1, 2). In so doing, bacteria often resort to mimicry by expressing proteins that adopt core functions of key actin cytoskeletal components, particularly actin filament nucleation and elongation factors. However, bacterial proteins tend to bypass the elaborate regulatory networks characteristic of their eukaryotic counterparts, offering a rare opportunity to dissect their functions within a simplified system (2, 3), with implications for our understanding of pathogenicity and the eukaryotic actin cytoskeleton alike.Rickettsiae are obligate intracellular Gram-negative pathogens that are transmitted to humans via arthropod vectors, such as ticks, fleas, and lice (4). Rickettsia species are responsible for a number of severe human diseases, including typhus and spotted fever (5). The spotted fever group, including Rickettsia parkeri, Rickettsia conorii, Rickettsia rickettsii, and over 20 other species throughout the world, uses the host-cell actin cytoskeleton to spread inter-and intracellularly. Similar to Listeria and Shigella, Rickettsia forms actin comet tails to propel its movement. However, the actin tails of Rickettsia consist of long and unbranched actin filaments, whereas those of Listeria and Shigella contain shorter and densely branched filaments (6, 7). These morphological differences stem from different molecular mechanisms for comet tail formation by these pathogens. Listeria and Shigella rely heavily on the activity of the host Arp2/3 complex that localizes uniformly along their tails (6). Although the Arp2/3 complex, activated by either host nucleation promoting factors (8) or the Rickettsia surface protein RickA (9, 10), is necessary for Rickettsia invasion (11), it is absent from Rickettsia tails (6). Another protein, Sca2 (surface cell antigen 2), has ...