THE CHAPERONE-ASSISTED MEMBRANE RELEASE AND FOLDING PATHWAY IS SENSEDBY 2 SIGNAL-TRANSDUCTION SYSTEMS

The assembly of interactive protein subunits into extracellular struct ures, such as pilus fibers in the Enterobacteriaceae, is dependent on the activity of PapD-like periplasmic chaperones. The ability of PapD to undergo a beta zippering interaction with the hydrophobic C-terminu s of pilus subunits facilitates their folding and release from the cyt oplasmic membrane into the periplasm. In the absence of the chaperone, subunits remained tethered to the membrane and were driven off-pathwa y via non-productive interactions. These off-pathway reactions were de trimental to cell growth; wild-type growth was restored by co-expressi on of PapD. Subunit misfolding in the absence of PapD was sensed by tw o parallel pathways: the Cpx two-component signaling system and the of modulatory pathway.