AVAILABILITY OF THE FIBER SUBUNIT CSGA AND THE NUCLEATOR PROTEIN CSGBDURING ASSEMBLY OF FIBRONECTIN-BINDING CURLI IS LIMITED BY THE INTRACELLULAR CONCENTRATION OF THE NOVEL LIPOPROTEIN CSGG

Curli, an adhesive surface fibre produced by Escherichia coil and salm onellae, was proposed on the basis of genetic evidence to follow a dis tinct assembly pathway involving an extracellular intermediate of the fibre subunit CsgA, the polymerization of which can be induced at the cell surface by a 'nucleator' protein (CsgB), Here we show biochemical ly that CsgA is actively secreted to the extracellular milieu and that CsgB is surface located, We demonstrate that the putative curli assem bly factor CsgG is an outer membrane-located lipoprotein. CsgG is high ly resistant to protease digestion both in vivo and in vitro. During c urli assembly, CsgG is required to maintain the stability of CsgA and CsgB, In line with this, it is possible to modulate the steady-state l evels of CsgA and CsgB by varying intracellular levels of CsgG, This s uggests that, in the absence of CsgG, CsgA and CsgB are proteolyticall y degraded, Moreover, curli production and steady-state levels of CsgA and CsgB can be increased above wild-type levels by overexpression of CsgG, meaning that the quantity of assembled curli fibres can be cont rolled by this lipoprotein.