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New insight into the molecular control of bacterial functional amyloids.

Taylor JD, Matthews SJ - Front Cell Infect Microbiol (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Sciences, Faculty of Natural Sciences, Imperial College of Science, Technology and Medicine London, UK.

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.

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Related in: MedlinePlus

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.
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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.

Mentions: The central component of the curli system, CsgG, acts as the membrane channel for secretion (Loferer et al., 1997; Robinson et al., 2006). Despite early reports of CsgA-related specificity, in isolation CsgG allows both inward and outward flux of macromolecules below a certain size and can thus be considered an ungated nanopore (Robinson et al., 2006; Taylor et al., 2011). Due to the absence of ATP within the periplasm and lack of chemical gradients, the energy source for efficient secretion of curli subunits is not known. CsgG is targeted to the outer membrane by the Lol transport system during which it is lipidated at its mature, N-terminal cysteine prior to outer-membrane insertion (Loferer et al., 1997). Although mutation of the cysteine prevents insertion of CsgG into the outer-membrane, it is able to fold independently within the periplasm (Goyal et al., 2013, 2014). This “soluble” species could be purified in the absence of detergent and its crystallization resulted in a high resolution structure of CsgG, solved using standard seleno-methionine phasing methods (Figure 1A). In this state, CsgG crystallized as two ring-shaped octamers arranged in D8 symmetry, interacting in a tail-to-tail fashion, with no transmembrane strands or helices apparent. Remaut and colleagues referred to the CsgG C8 configuration as a “pre-pore” state since it resembles a class of pore-forming proteins (PFPs) that oligomerise prior to membrane insertion (Iacovache et al., 2010).


New insight into the molecular control of bacterial functional amyloids.

Taylor JD, Matthews SJ - Front Cell Infect Microbiol (2015)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4389571&req=5

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.
Mentions: The central component of the curli system, CsgG, acts as the membrane channel for secretion (Loferer et al., 1997; Robinson et al., 2006). Despite early reports of CsgA-related specificity, in isolation CsgG allows both inward and outward flux of macromolecules below a certain size and can thus be considered an ungated nanopore (Robinson et al., 2006; Taylor et al., 2011). Due to the absence of ATP within the periplasm and lack of chemical gradients, the energy source for efficient secretion of curli subunits is not known. CsgG is targeted to the outer membrane by the Lol transport system during which it is lipidated at its mature, N-terminal cysteine prior to outer-membrane insertion (Loferer et al., 1997). Although mutation of the cysteine prevents insertion of CsgG into the outer-membrane, it is able to fold independently within the periplasm (Goyal et al., 2013, 2014). This “soluble” species could be purified in the absence of detergent and its crystallization resulted in a high resolution structure of CsgG, solved using standard seleno-methionine phasing methods (Figure 1A). In this state, CsgG crystallized as two ring-shaped octamers arranged in D8 symmetry, interacting in a tail-to-tail fashion, with no transmembrane strands or helices apparent. Remaut and colleagues referred to the CsgG C8 configuration as a “pre-pore” state since it resembles a class of pore-forming proteins (PFPs) that oligomerise prior to membrane insertion (Iacovache et al., 2010).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Sciences, Faculty of Natural Sciences, Imperial College of Science, Technology and Medicine London, UK.

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.

Show MeSH
Related in: MedlinePlus