Amyloid by Design: Intrinsic Regulation of Microbial Amyloid Assembly

Abstract

The term amyloid has historically been used to describe fibrillar aggregates formed as the result of protein misfolding and that are associated with a range of diseases broadly termed amyloidoses. The discovery of "functional amyloids" expanded the amyloid umbrella to encompass aggregates structurally similar to disease-associated amyloids but that engage in a variety of biologically useful tasks without incurring toxicity. The mechanisms by which functional amyloid systems ensure nontoxic assembly has provided insights into potential therapeutic strategies for treating amyloidoses. Some of the most-studied functional amyloids are ones produced by bacteria. Curli amyloids are extracellular fibers made by enteric bacteria that function to encase and protect bacterial communities during biofilm formation. Here we review recent studies highlighting microbial functional amyloid assembly systems that are tailored to enable the assembly of non-toxic amyloid aggregates.

Keywords: functional amyloid; microbial amyloids; nature-inspired therapeutics; protein misfolding; toxic oligomer.

Figure 1

(A) Kinetics of amyloid polymerization can be monitored with ThT fluorescence [24]. Amyloid polymerization occurs via a rate-limiting lag phase, growth or elongation phase and stationary phase. (B) Mechanisms of amyloid polymerization. In (I) primary nucleation, protein monomers form a minimal nucleus competent for elongation [25]. (II) Elongation occurs when monomers add on to an existing fibril. (III) Surface-catalyzed nucleation and (IV) fragmentation describe two modes of secondary nucleation [–28]. In surface-catalyzed nucleation, new fibrils are nucleated on the surface of an existing fibril in a monomer-dependent process [26,27]. Fragmentation occurs when an existing fibril breaks to produce two new independent fibrillar units capable of undergoing elongation [28].

Figure 2

The csg operons encode the curli-specific genes [52]. CsgA is the major curlin subunit and is secreted into the periplasmic space in an unstructured conformation via SecYEG [3]. CsgA is maintained as unstructured in the periplasmic space by the amyloid inhibitor, CsgC [13]. Soluble CsgA is secreted to the outer membrane via the nonameric CsgG pore and nonameric CsgE adaptor complex [58,59,63]. CsgA assumes a β-sheet rich conformation upon interaction with the minor curlin subunit and nucleator protein, CsgB, which is tethered to CsgG via CsgF [11,56,60,62].

Figure 3

(A) The proposed structure of CsgA in the amyloid form assumes a β-helical conformation with a rectangular core [3,12,64,65,79,106]. (B) HET-s(218–289) forms a β-helical structure by stacking sets of ‘pseudo-repeats’ [81,89,90]. (C) The amyloid formed from PSMα3 peptide assumes a ‘cross-α’ structure [100].