Availability of the fibre subunit CsgA and the nucleator protein CsgB during assembly of fibronectin-binding curli is limited by the intracellular concentration of the novel lipoprotein CsgG.

Curli, an adhesive surface fibre produced by Escherichia coli and salmonellae, was proposed on the basis of genetic evidence to follow a distinct 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 biochemically that CsgA is actively secreted to the extracellular milieu and that CsgB is surface located. We demonstrate that the putative curli assembly factor CsgG is an outer membrane-located lipoprotein. CsgG is highly resistant to protease digestion both in vivo and in vitro. During curli assembly, CsgG is required to maintain the stability of CsgA and CsgB. In line with this, it is possible to modulate the steady-state levels of CsgA and CsgB by varying intracellular levels of CsgG. This suggests that, in the absence of CsgG, CsgA and CsgB are proteolytically 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 controlled by this lipoprotein.