Curli play critical roles in biofilm formation, host cell adhesion, and colonization of inert surfaces in many Enterobacteriaceae. In Escherichia coli, curli biogenesis requires 7 curli-specific gene (csg) products-CsgA through G-working in concert. Of them, CsgG and CsgF are 2 outer membrane (OM)-localized components that consists of the core apparatus for secretion and assembly of curli structural subunits, CsgB and CsgA. Here, we report the cryogenic electron microscopy (cryo-EM) structure of CsgG in complex with CsgF from E. coli. The structure reveals that CsgF forms a stable complex with CsgG via a 1:1 stoichiometry by lining the upper lumen of the nonameric CsgG channel via its N-terminal 27 residues, forming a funnel-like entity plugged in the CsgG channel and creating a unique secretion channel with 2 constriction regions, consistent with the recently reported structure of the CsgG-CsgF complex. Functional studies indicate that export of CsgF to the cell surface requires the CsgG channel, and CsgF not only functions as an adaptor that bridges CsgB with CsgG but also may play important roles in controlling the rates of translocation and/or polymerization for curli structural subunits. Importantly, we found that a series of CsgF-derived peptides are able to efficiently inhibit curli production to E. coli when administrated exogenously, highlighting a potential strategy to interfere biofilm formation in E. coli strains.

Coat complexes coordinate cargo recognition through cargo adaptors with biogenesis of transport carriers during integral membrane protein trafficking. Here, we combine biochemical, structural, and cellular analyses to establish the mechanistic basis through which SNX27-Retromer, a major endosomal cargo adaptor, couples to the membrane remodeling endosomal SNX-BAR sorting complex for promoting exit 1 (ESCPE-1). In showing that the SNX27 FERM (4.1/ezrin/radixin/moesin) domain directly binds acidic-Asp-Leu-Phe (aDLF) motifs in the SNX1/SNX2 subunits of ESCPE-1, we propose a handover model where SNX27-Retromer captured cargo proteins are transferred into ESCPE-1 transport carriers to promote endosome-to-plasma membrane recycling. By revealing that assembly of the SNX27:Retromer:ESCPE-1 coat evolved in a stepwise manner during early metazoan evolution, likely reflecting the increasing complexity of endosome-to-plasma membrane recycling from the ancestral opisthokont to modern animals, we provide further evidence of the functional diversification of yeast pentameric Retromer in the recycling of hundreds of integral membrane proteins in metazoans.