New insight into the molecular control of bacterial functional amyloids

Abstract

Amyloid protein structure has been discovered in a variety of functional or pathogenic contexts. What distinguishes the former from the latter is that functional amyloid systems possess dedicated molecular control systems that determine the timing, location, and structure of the fibers. Failure to guide this process can result in cytotoxicity, as observed in several pathologies like Alzheimer's and Parkinson's Disease. Many gram-negative bacteria produce an extracellular amyloid fiber known as curli via a multi-component secretion system. During this process, aggregation-prone, semi-folded curli subunits have to cross the periplasm and outer-membrane and self-assemble into surface-attached fibers. Two recent breakthroughs have provided molecular details regarding periplasmic chaperoning and subunit secretion. This review offers a combined perspective on these first mechanistic insights into the curli system.

Keywords: Alzheimer's; Parkinson's; amyloid; biofilm; chaperone; curli; secretion.

Figure 1

The structure of CsgG. (A) CsgG C16S mutant crystallized as an octamer with the transmembrane region buried or disordered. One protomer is colored in a gradient (N-terminus = blue to C-terminus = red). The lower panel shows the crystallographic symmetry encountered. (B) Wild type CsgG forms nonamers. The N-terminal lipidation sites are marked by spheres. Coloring as in (A). (C) The central pore loops across which curli subunits pass is displayed by slicing through the center of the pore. Highly conserved side-chains are shown as sticks. (D) Surface representation of wild type CsgG, viewed from the exterior of the cell. The N-terminal ~20 residues wrap around the adjacent protomer. (E) Structural alignment between CsgG monomers from the “pre-pore” state (yellow) and membrane-inserted state (red helices, blue strands, gray loops). The RMSD is 0.85 Å.

Figure 2

The three stages of curli fiber biogenesis. (A) Secretion system assembly: CsgE and CsgG achieve the translocation of CsgF, which folds and binds CsgG. Loss of CsgE results in a dramatic reduction in surface display of CsgF. (B) Pre-secretion subunit handling: Incoming CsgA or CsgB monomers interact with CsgC, which delays spontaneous formation of toxic oligomers. It is expected that subunits within this CsgC-buffered pool (signified by the dashed circle) are either secreted, digested or revert to an amyloidogenic pathway. (C) Subunit secretion, nucleation and fiber assembly: Each curli subunit encounters CsgE (nonamers?) and becomes trapped within the periplasmic cavity of CsgG. Partially folded subunits then traverse the central pore and are released into the extracellular milieu. The folding pathway of periplasmic CsgA determines the appearance and properties of extracellular fibers, thus it is unlikely that CsgA is secreted as a linear polypeptide. Once outside the cell, CsgB is interacts with CsgF and initiates nucleation of the CsgA fiber.