Functional amyloid in a lipid-like environment: a merry dance of many steps

Functional amyloid, which unlike its pathological counterpart serves a biological purpose, is produced in a carefully orchestrated sequence of events. In bacteria, the major amyloid component is transported over the periplasm and through the outer membrane to assemble on the bacterial cell surface. During its life time, the amyloid protein may be exposed to both membrane lipids and extracellular surfactant, making it relevant to study its interactions with these components in vitro. Particularly for charged surfactants, the interaction is quite complex and highly dependent on the surfactant:protein molar ratio. Low ratios typically promote aggregation, likely by binding the proteins to micelles and thus increasing the local concentration of proteins, while higher concentrations see an inhibition of the same process as the protein is diluted out and immobilized on individual micelles. This is particularly pronounced for strongly anionic surfactants like SDS; the naturally occurring biosurfactant rhamnolipid interacts more weakly with the protein, which still not only allows aggregation but also leads to less detrimental effects at higher ratios. Similarly, anionic vesicle-forming lipids largely stimulate aggregation likely because of weaker interactions. Anionic lysolipids, thanks to their micelle-forming properties, resemble SDS in their impact on fibrillation. There are also examples of systems where membrane binding sequesters an otherwise amyloidogenic sequence and prevents fibrillation or-quite the opposite- liberates another part of the protein to engage in self-assembly. Thus, membranes and surfactants have very varied roles to play in the biogenesis and function of bacterial amyloid.