LAUSR

In this work, the authors show that the phase separation of a cell surface-associated protein called CsgF is critical to mediate the secretion and assembly of the amyloidogenic curli subunits. Curli are functional amyloids present on the outer membrane of E. coli . CsgF is required for the proper assembly of curli. Here, we found that the CsgF phase separates in vitro and that the ability of CsgF variants to phase-separate is tightly correlated with CsgF function during curli biogenesis. Substitution of phenylalanine residues in the CsgF N-terminus both reduced the propensity of CsgF to phase-separate and impaired curli assembly. Exogenous addition of purified CsgF complemented csgF  ^− cells. This exogenous addition assay was used to assess the ability of CsgF variants to complement csgF  ^ ‒ cells. CsgF on the cell surface modulated the secretion of CsgA, the curli major subunit, to the cell surface. We also found that the CsgB nucleator protein can form SDS-insoluble aggregates within the dynamic CsgF condensate. We propose that these multicomponent CsgF-B condensates form a nucleation-competent complex that templates CsgA amyloid formation on the cell surface.