NUCLEATOR FUNCTION OF CSGB FOR THE ASSEMBLY OF ADHESIVE SURFACE ORGANELLES IN ESCHERICHIA-COLI

Curli are surface organelles in Escherichia coli that assemble outside the bacterium through the precipitation of secreted soluble CsgA mono mers, requiring the CsgB nucleator protein, Using immunoelectron micro scopy and immunoblotting assays, CsgB is shown to be located on the ba cterial surface and also as a minor component of wild-type curli, CsgB lacking its 20 N-terminal residues when fused to maltose-binding prot ein (MBP) can still trigger polymerization of CsgA monomers in vivo, H owever, the resulting surface organelles are only formed at one of the two bacterial poles and are morphologically distinct from wild-type c urli, These Bfco organelles (CsgB-Free Curli-related Organelles) are h ighly regular structures reacting with anti-CsgA, but not anti-CsgB an tibodies, The CsgB of the active MBP-CsgBII fusion is surface exposed but, unlike the native CsgB in wild-type curli, is not detectable in t he Bfco organelles. Overexpression of csgB within a csgA mutant result s in the formation of short CsgB polymers on the cell surface, It is s uggested that in wild-type bacteria, both CsgA and CsgB are secreted p roteins. Interaction between CsgA and CsgB triggers wild-type curli fo rmation, resulting in CsgA-CsgB heteropolymers, while surface-anchored CsgB in MBP-CsgBII triggers morphologically distinct, CsgB-free/CsgA Bfco organelles. In the absence of CsgA, CsgB can self-assemble into p olymers.