Abstract

Many bacteria produce functional amyloid, i.e. proteins which are secreted through dedicated export systems and self-assemble on the bacterial surface, often with the help of nucleator proteins. These amyloid proteins serve a diversity of purposes, but the most prominent one appears to be structural stability; for example, overexpression of the amyloid-forming protein FapC in Pseudomonas species strengthens bacterial biofilm against mechanical insults, increases hydrophobicity and protects against desiccation. Similar properties are ascribed to the curli-forming protein CsgA from E. coli. Unlike pathological amyloid, functional amyloid has been under evolutionary pressure to self-assemble efficiently (i.e. in a single “fast track”) to very stable higher-order structures. My lab is engaged in an effort to understand the molecular basis for these properties in more detail and I will present recent progress in this area.

Both FapC and CsgA consist of multiple imperfect repeats. Stepwise removal of these repeats gradually changes the preferred fibrillation pathway of FapC from a mechanism dominated by primary nucleation (the simplest pathway) to one in which fragmentation plays an increasing role; further, they significantly destabilize the fibrils (as measured by their ability to resist dissolution in formic acid). Chemical deanturants such as urea slow down fibrillation by several fold but CsgA can still fibrillate efficiently at 6M urea, testimony to its remarkable stability. In contrast to this, naturally occurring biosurfactants (rhamnolipids) and outer membrane components (lipopolysaccharide) markedly stimulate fibrillation. Nevertheless, fibrillation can be inhibited by naturally occurring fibrillation inhibitors, including plant polyphenols like epigallo catechin gallate (EGCG) and penta-galloyl gallate (PGG), which direct FapC to off-pathway oligomeric structures that eventually forms amorphous aggregates. Thus despite their strong drive to self-assemble, functional amyloid are still sensitive to the same type of manipulation as their pathological counterparts.