Abstract
Amyloids are fibrillar protein aggregates with an ordered spatial structure called “cross-β”. While some amyloids are associated with development of approximately 50 incurable diseases of humans and animals, the others perform various crucial physiological functions. The greatest diversity of amyloids functions is identified within prokaryotic species where they, being the components of the biofilm matrix, function as adhesins, regulate the activity of toxins and virulence factors, and compose extracellular protein layers. Amyloid state is widely used by different pathogenic bacterial species in their interactions with eukaryotic organisms. These amyloids, being functional for bacteria that produce them, are associated with various bacterial infections in humans and animals. Thus, the repertoire of the disease-associated amyloids includes not only dozens of pathological amyloids of mammalian origin but also numerous microbial amyloids. Although the ability of symbiotic microorganisms to produce amyloids has recently been demonstrated, functional roles of prokaryotic amyloids in host–symbiont interactions as well as in the interspecies interactions within the prokaryotic communities remain poorly studied. Here, we summarize the current findings in the field of prokaryotic amyloids, classify different interspecies interactions where these amyloids are involved, and hypothesize about their real occurrence in nature as well as their roles in pathogenesis and symbiosis.
Highlights
The term “amyloid” dates back to the previous century
This structure is typical of amyloids, yet there is no rule without exception: phenol-soluble modulin α3 (PSMα3), a secreted protein of Staphylococcus aureus, forms fibrils possessing physicochemical properties of amyloids but shows “cross-α” diffraction pattern that corresponds to perpendicularly stacked α-helices rather than to β-sheets in “cross-β” structure [8]
Prokaryotic amyloids identified to date contribute to various interspecies interactions, including Type I interactions between pathogenic bacteria and multicellular hosts, Type II interactions between different microbial species in the communities, and Type III host–symbiont interactions
Summary
The term “amyloid” dates back to the previous century. In 1838, Matthias Schleiden introduced “amyloid” (from Latin “amylum”—starch) to describe a starch material in plant cells [1]. The spatial organization of amyloid fibrils determines “cross-β” diffraction pattern characterized by two scattering diffraction signals: ~4.7 Å, corresponding to the interstrand distance, and ~10 Å, corresponding to the distance between β-sheets [6,7] This structure is typical of amyloids, yet there is no rule without exception: phenol-soluble modulin α3 (PSMα3), a secreted protein of Staphylococcus aureus, forms fibrils possessing physicochemical properties of amyloids but shows “cross-α” diffraction pattern that corresponds to perpendicularly stacked α-helices rather than to β-sheets in “cross-β” structure [8]. Amyloids have been shown to exhibit “apple-green” birefringence in polarized light upon CR binding [19] and demonstrate specific fluorescence emission spectra [20]. SDS (Sodium dodecyl sulfate)-resistant polymer formation, fibrils on the cell surface, binding anti-RopA antibodies
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