Figure 6

Models of the ClpXP-catalysed degradation of knotted fusion proteins. Trefoil knotted protein (orange), ATP-dependent protease (blue/red), highly stable ThiS ‘plug’ domain (green). (A) Knot tightening and abutting the stable ThiS domain outside of the ClpXP machinery. In this model, once the YbeA domain has been unfolded, the knot slips along the chain as translocation takes place until it reaches the ThiS domain at which point it can go no further. The knot does not enter the translocation pore. (B) The tightened knot enters the translocation pore and when it reaches the proteolytic active sites it is hydrolysed. The chain continues to translocate and be degraded until the stable ThiS domain abuts the surface of ClpXP. (C) The tightened knot slips along the polypeptide chain until it can slip no further as it abuts the ThiS domain. At this point, the knot can enter the translocation pore and part of the knotted region reaches the proteolytic sites and is hydrolysed.