Fig. 4

Model for trigger factor-assisted force transmission through a molecular pore. a A polypeptide chain exiting a molecular tunnel is under some tension due to the confined geometry (left). When the protein folds in the edge of the tunnel (right), it pulls out a fraction of the polypeptide in the channel (purple fragment), increasing the mechanical tension and hampering the folding transition. b When a polymer is confined in a tube of fixed length L _tunnel is subject to an effective force, which depends on its contour length L _contour. This force can be approximately modeled by a freely jointed chain (FJC) model, where the end-to-end distance is fixed to L _tunnel and the force depends on the length of the confined polymer L _contour (blue line). Accordingly, the folding probability with (red) and without (black) TF of a polypeptide chain in such scenario depends on L _contour. c Protein folding generates shortening which increases the tension of the polymer, transmitting a force through the tunnel. We can estimate this expected force as the product between the force generated by the confined polymer and the probability of folding at such length L _contour. This is calculated with (red) and without (black) TF revealing that there is an optimal length of the confined polymer, between 3 and 4 times the length of the tunnel. Shorter polymers would induce large tensions that would avoid protein folding, so the expected force is zero. Longer polymers would lead to small values of the expected force. In the optimal region, TF boosts the transmitted force by almost 4 pN (purple)