Amyloid Formation in Bacteria
- 226 Downloads
Amyloids result from the structural conversion of soluble proteins into self-associating conformations able to assemble into macromolecular insoluble fibrillar aggregates. The formation of amyloids is the underlying cause of incurable human disorders like Alzheimer’s or Parkinson’s diseases or certain cancers. However, these highly ordered protein assemblies also form in the simplest organisms, like yeast or bacteria (de Groot et al. 2009). Common limitations of recombinant protein production in bacteria are misfolding and the concomitant formation of insoluble protein aggregates known as inclusion bodies (IBs). The formation of IBs in bacteria is now seen as part of a general cellular response related to the presence in the cell of unfolded proteins and as a pathway for the control of aggregation. These intracellular aggregates are dense, porous, hydrated, apparently amorphous, and refractile particles of nearly 1 μm in diameter and are commonly localized at the cell poles. The mechanism of IBs assembly in Escherichia coli has been characterized in detail and shown to be nucleation driven and sequence specific. Transmission electron microscopy and atomic force microscopy allowed the visualization of the presence of fibrillar structures coexisting with amorphous material inside the bacterial IBs formed by different proteins. Circular dichroism, infrared spectroscopy, X-ray diffraction, as well as solid and liquid state nuclear magnetic resonance data converge to indicate that bacterial IBs are characterized by the presence of an extensive intermolecular β-sheet structure similar at the molecular level to the characteristic cross-β-sheet structure of amyloid fibrils (Villar-Pique et al. 2015). Accordingly, bacterial aggregates bind with good affinity to amyloid diagnostic dyes like Thioflavin-T and Congo red. In IBs, ordered aggregated polypeptides can coexist with unstructured species or even with functional globular domains; indeed the transition from a native to an amyloid structure seems to occur within the aggregate itself. In addition to fibrillar material, the presence of SDS-resistant oligomeric assemblies has been detected in the insoluble fraction of bacteria. Therefore, it is not surprising that bacterial aggregates affect cell division and aging, can induce cytotoxicity in neuronal cells, and become infectious in the case of IBs formed by prion proteins (Villar-Pique and Ventura 2012). Indeed, the IBs formed by mammalian prion proteins in bacteria recapitulate the strain phenomena, in which a single sequence is able to render aggregates with different structural and functional properties (Macedo et al. 2016). Overall, the amyloid-like properties of bacterial aggregates suggest that these organisms might provide a simple but biologically relevant background for understanding pathologic protein deposition.
The increasing medical and economic impact of aggregation-linked diseases in our society has fueled the development of methods to identify chemical compounds that can interfere with amyloidogenic pathways. In this context, bacterial cell factories can be easily adapted to develop in vivo screening tools for amyloid aggregation inhibitors that will outperform the conventional in vitro screening procedures used by the industry (Navarro et al. 2016).
Interestingly, the bacterial ability to construct amyloid structures is not restricted to recombinant IBs, and an increasing number of unrelated natural bacterial polypeptides able to form functional amyloid fibrils are being described. Bacteria exploit the unique mechanical properties of the macromolecular assemblies formed by these proteins for different biological functions. Bacterial amyloids are usually extracellular fibers that extend from the cell surface. In this way, Escherichia coli use curli amyloid-like fibers to interact with host tissues, for biofilm formation, and to evade the immune system. Other bacterial amyloids involved in biofilm formation are TAFI, the curli homolog in Salmonella, FapC in many Pseudomonas species, TasA in Bacillus subtilis, the phenol-soluble modulins in Staphylococcus aureus, and the adhesin protein P1 in Streptococcus mutans in dental plaque biofilms (Taglialegna et al. 2016). Bacteria also exploit the inherent toxicity of amyloid assemblies for its own benefit, as in the case of the toxin microcin E492 produced by Klebsiella pneumonia, which oligomerizes into the cytoplasmic membranes of Enterobacteriaceae, causing pores that result in cell death. The use of the amyloid fold for functional purposes in bacteria requires both the contribution of the protein homeostasis machinery and the evolution of complex dedicated secretion pathways to avoid the intracellular accumulation of toxic species that can kill the cell (Van Gerven et al. 2015). Indeed, blockage of these protective systems is now seen as a promising strategy to fight pathogenic bacteria. However, the discovery of the first prion protein, the Rho terminator factor of Clostridium Botulinum, indicates that functional amyloids can also occur in the interior of bacteria, acting as an epigenetic inheritance bet-hedging strategy which aimed to promote phenotypic diversity, likely contributing to rapid adaptation of bacteria to fluctuating environments and enhancing the evolution of new traits and the persistence of bacterial infections (Pallares and Ventura 2017).