Cell surface-localized CsgF condensate is a gatekeeper in bacterial curli subunit secretion

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.

Automated summary

The authors demonstrate that the phase separation of the cell surface-associated protein CsgF is crucial for the secretion and assembly of amyloidogenic curli subunits. CsgF is important for the proper assembly of curli, and its ability to phase-separate is correlated with its function. The formation of multicomponent CsgF-B condensates is proposed to facilitate amyloid formation on the cell surface.