Mol. Cell 10.1016/j.molcel.2014.12.025

Credit: ELSEVIER

Many neurodegenerative diseases are associated with protein misfolding, which leads to the formation of ordered aggregates of proteins called amyloid fibers. Amyloid fibers can also be formed as a normal, functional process unrelated to misfolding. For instance, the Escherichia coli CsgA protein is secreted from cells in an unpolymerized form and assembles into extracellular amyloid called curli, which are important in surface attachment and biofilm formation. Although the factors required for curli fiber formation have been identified, it has been unclear how curli formation is prevented inside the cell, where it can be cytotoxic. To address this question, Evans et al. tested components of the csg operons for inhibitory activity in vitro by generating periplasmic extracts (PEs) from csg mutants. PEs from cells lacking CsgE, a chaperone required to direct CsgA to CsgG for secretion, could inhibit CsgA amyloid assembly in vitro, whereas PEs from cells lacking CsgC could not. Fractionation of the PE from ΔcsgG cells—which could inhibit assembly—as well as an intact-cell curli assay implicated CsgC in inhibiting CsgA amyloid formation. When CsgC was deleted, CsgA aggregates were intracellular, cell viability was compromised, and cellular stress associated with the abnormal accumulation of misfolded proteins was induced. CsgC could inhibit CsgA amyloid formation in vitro, as detected by far-UV CD spectroscopy and NMR, presumably by stabilizing a preamyloid intermediate of CsgA, as detected by an amyloid conformation-specific antibody. Although the authors could not detect a stable interaction between CsgA and CsgC by native gel electrophoresis, they were able to map the regions of CsgA through which CsgC acts using the in vitro assay. Finally, they found that E. coli CsgC could inhibit amyloid formation of CsgA from Salmonella enterica and Citrobacter koseri as well as human α-synuclein, which shares an 8-amino-acid motif with CsgA. Having a dedicated amyloid inhibitor may be what distinguishes functional amyloid from disease-associated amyloid.